There is no concrete rule that you should never mix narrative POVs, but these decisions should be made intentionally. In general, sticking to one POV is the common advice in writing.
His name don't cross my mind right now, but there's at least one modern book author who write all his stories with an alternation between first person POV and third person POV, one chapter for each. But as you said, it's intentional. It's his style signature, and it works fine because he already know the story will be wrote this way when he's still just letting it grow in his mind.
By opposition to the (too many) devs who seem to pass from one POV to the other for random reasons. Bonus points if they stick to a first person camera, but have third person dialogs that show the internal thoughts of the non-MC characters.
I'd say that poor writers tend to mainly stick to dialogues and internal thoughts of various characters because they do not have the writing skills to describe scenes or situations in exquisite detail using narration.
Strictly speaking, it's not just a question of writing. Whatever if it's 2D or 3D CGs, finding, then effectively making, the right pose and expression to fit the situation isn't something easy. And it obviously become more difficult when you've oppose this to the dialog lines.
- What she's saying disagree with MC's intent, I'll make her grumpy.
- But wait, secretly she's agreeing, I need to make her happy.
- Stop, if I make her happy, it will feel totally odd in regard of what she say.
- How the fuck do one make a grumpily-happy or happily-grumpy expression ?
1st person narration: I see her blushing as she looks down at my crotch.
More, "wait, has she blushed while looking at my crotch ?"
A first person narration should be interrogative when it's about others reaction. Reading other's thoughts through their expression and behavior is something difficult. Any average human only assume that it can be what happened, but there's always a part of doubt. It's only if the said reaction stay for a long enough time ("yes, it's clearly why she's blushing"), or happen more than once ("she was, and she's doing it again") that it can become affirmative. Because it's a confirmation of our assumption.
Take the classical "you're in a half crowded place, and a girl looks in your direction and smile. Is she smiling because she find you charming, or because at your left, or right, there's a good old friend of hers ? It's only times that will tell you this. Either because her eyes will follow you when you'll move, or because the said friend will pass you and react in return.
No narration: Show this reaction with just renders. Or indeed, you could just write the non-MC thoughts.
It's here that it become tricky. Not all situation can be expressed through a first person POV camera ; or more precisely, can expressed in an obvious enough way.
Encouraged by the reaction of her mother, the MC decide to let his hand slightly brush her ass when she leave. Even if you overdo it, and make the MC looks right at his mother ass to show what is happening, the mother is turning her back to the MC. This mean that, by default (first person Camera and narration) you can't show her reaction.
Then you've to trick, making the scene happen in a room where there's a mirror, and placing the characters in such way that the mirror will show what's the reaction of the mother. Or you can use the sister as intermediary ; she's entering the room at this moment, and once the mother have left say something like, "I don't know what you where talking about, but I assume that for once you haven't be a dick, since mom was smiling".
And obviously, this isn't always possible, what mean that you've to plan your story beforehand not just in regard of the situation, but also in regard of their context. If something happen, whatever this something, you've to know before you start to write the scene and build the CGs, that you'll be able to show the key moment instead of finding yourself in such position that you have to fall back to the narration or non-MC internal thoughts.
You could certainly have internal thoughts of the MC and others while still having narration in places.
MC's internal thoughts are something different. If you rely mostly on first person camera, you've no way to show them. Plus, depending of the context, they can make perfect sens.
A game like
Shattered, by example, make an intensive use of MC's thoughts. But the said MC is half psychotic and half depressive, therefore it fit the personality and don't feel wrong. Same for
The DeLuca Family's MC, who was an average Joe, and the day after was forced to be part of a mafia family, climb his way out of it, while having a target on his back (so far he survived three assassination attempts). It's clearly a context in which anyone would pass a lot of time thinking about a way to save his ass.
But at the opposite, the average "wanna fuck them all" MC have no reason to be so lost in thoughts. It can only works if you want to express a dichotomy between his desire (be the Alphaest alpha in the town) and his personality (good mama boy who would put his vest on the ground for his princess to not wet her shoes). But it's not what most of those MC are supposed to be. The game sell them as guys of actions, therefore you shouldn't make them pass so much time reflecting about themselves and the others.
To use your "mama looks at my crotch" example, such MC wouldn't thought, nor question the mothers attitude. Assuming that he isn't totally stupid, he would assume that his mother like to looks at his crotch and place himself in such way that she'll have a better view. And if it's not the first time, or if he's totally stupid, he would also be bold, asking her if she like the view.
What lead to the main reason why so many games rely on internal thoughts: The lack of psychology background of their author.
The say instead of showing, because they can't put themselves on the shoes of the characters, and therefore don't know how they would react in such situation. It's why, in place of a blushing mother with an embarrassed expression, they make her think "my son's dick is sooooo big". And why instead of the alpha MC they want to depict taking action, they write beta MC who pass most of their time reflecting about what the others are thinking.
But this isn't a fatality. They should just sit back on their chair, and let their mind wander in their past. We all seen persons embarrassed by a situation. We all witnessed people being too bold. They just need to remember those memories, to find the information they are missing.
And with time, it will become easier, because it will pass from a second hand knowledge ("this particular time, she reacted like this") to a conscious knowledge ("face to such situation, she'll react this way").
