We're encountering a bug in Unity where Animations containing a drawing swap in the animation sometimes chooses to not reset when blending into the next animation.
For example, let's lay out two animations:
1) an IDLE where the character has a smile on their face. There are keys at the beginning and end of the animation stating this smile should be on for the entire duration of the idle animation.
2) an ATTACK where the character has some teeth gritted mouth on their face, but the animation contains a key flagging the SMILE attachment ON, then a few frames in, a flag for the angry mouth to swap in, then a flag at the very end of the whole animation for the angry mouth to stay on this character.
In theory, there are keys establishing that these swaps should occur no matter what, and that even if a new animation starts up, we have "bookend-ed" any appropriate drawings we want the animation to start with.
The PROBLEM we are seeing, is sometimes, only sometimes, using the example above, the angry face will remain on the character when transitioning to the idle again - despite the fact we have a key establishing that at the beginning of the idle, we want that character smiling. It might reset to the appropriate drawing if it transitions to another animation.
So we've had this issue many times in the past, (not just with drawing swaps, actually) and we've concluded that bookends for any bone and anything being keyed in an animation really is the only way to avoid a break in blending of animations. We still encounter this issue from time to time and it's sorta boggling our minds - I can't decide if it's an issue with the blend system or how we're keying animations or what. Generally, our solution has been a seemingly neanderthal approach by just adding a bunch of keys for the attachments to reinforce "YES, we want this drawing to be the visible drawing at this point in the animation!". I'm hoping someone has a better approach or idea as to how to solve this problem.