Tags have always been accurate only to a certain extent, probably in every game here you can find questionable tags or missing ones. But for now, what is in the game is hard to call cheating. In fact, MC is not in an "official" relationship, even with Bella, there is something like testing the waters so you're not actually cheating at this stage of relationships (although it would certainly feel like it). I think everything will change in the next season, and then the tag will appear.
The general rule is that you have freedom of choice, but not freedom from consequences.
There aren't any visible consequences yet if you kiss Bella, and then start making out with Vic and Mila at Vic's movie night, but the girls will notice that and tell you. And by the way, behind the scenes the game remembers most of your choices, so there's no telling which of your actions might bite your ass later.
I don't think Bella will take it easy that you're flirting with others, even though you kind of agreed to a casual relationship and no strings attached.