Ok so this bug has been happenning since the 1.16.0 now and maybe even before that. So the bug is that the achievements are never loading their textures. They are just a grey person and a ?. 

How to reproduce:

1. Start up Minecraft.
2. Wait for your skin to load. 
3. Go onto your profile.
4. Go onto Achievements.

Expected results: The achievements will have pictures.

Observed results: They just have a grey person and a ?.