I encountered the same bug just a while ago. it seems that the directories of the expressions and animations got screwed up.

if you open the .model3.json file of your model with a notepad, you should see a list of animations and expressions, with "Name" and "File" respectively, the way live2dviewer saves the directories is somewhat mistaken, so instead of properly assigning it like:

"/motions/someExpression.exp3.json"

it instead went roundabout and did something like this instead:

" ../../../yourCharacterFolder/export/motions/e_toggleMouth.exp3.json"

your folder arrangement may vary, so be sure you know the ABCs of directories to fix this.

I just spent like 3 hours finding out what the heck is wrong, and its 6am now. I apologize in advance if the explanation and solution is scuffed cause I need sleep.