>>368428554
I just uploaded a test version of mine with that particular layer removed, and I can confirm that the layer is how my avatar resets its expressions.
It also might sound wacky, but try to insert another keyframe in your animations with the same animation blendshapes or toggles in very short duration like pic rel. I've noticed that even with the default reset layer removed, the expressions that had the short duration reset back to the original shape, while the others did not.
Also, my avatar has its hand movements done in the "gesture" layer, not FX.

FYI, there's a short note on write defaults in vrc docs.
https://docs.vrchat.com/docs/avatars-30#write-defaults-on-states