I see two problems with this feature. I usually keep a game I am playing open until I finish the update. But what is shown isn't the play time but the time the game was open. Also if you have different playthrough it cumul. Good idea but poorly executed IMO
the overall time played in the main menu will accumulate yeah. but the time played in saves seems to show the specific time for the specific playthrough.
but yeah, it can't really differentiate if you just leave the game open, that's a you problem though. it's still good info to have even with that.
also, I used to leave the game open too, like you. but if the game crashes, then you can lose everything you progressed to since the last save (for the latest renpy versions) or even since you last opened the game. so I just quit when I leave now. at least, we have to save and go back to the main menu so we're sure everything is saved.