today i learned:
unity's "write defaults" setting on animations does almost the exact opposite of what i thought it does
what we thought it did was write things about the current animation back to the default state of the model, so for example, it wouldn't be wanted if moving a bone around since it might get stuck in the wrong place
what it actually does is look at everything else on the animation layer - it controls whether everything on that layer not being currently animated is set back to defaults or kept with the same value as it previously had
and now i see the cause of a few past weird problems with my model, and why a lot of vrchat advice tends to be a bit... superstitious about it; it's conceptually easier to just keep it consistent even if that sometimes means more work