majority of people don’t use tweening, tweening is helpful for beginner animators such as yourself, but it isn’t really a tool that is meant to be used long term (in my opinion), and because most people don’t animate with tweening, it also doesn’t make much sense to expect movieclips to work well with it.

plus it can depend on whether the original animator used tweening when creating the movieclip or not, and considering most movieclips are effects, most don’t lol.

I can’t really explain properly because i’m not Ralph nor do I actually know the programming behind SN but yea

I use Tween in most of my animations, for example, a test animation of the Wall Dweller.