hello,
apparently that pesky Unity bug
that was plaguing avatar creators trying to animate material changes on more complex avatars (more details in the link) was backported to v2019.4.39f1.
i can confirm it now works fine in v2019.4.40f1.
is there any chance of VRC getting a small update to v2019.4.39f1, or whatever the newest v2019.* will be at the time?