Stickers appear to render at a render queue higher than avatars can render at, which means that they'll render over any (ZWrite off) transparency, and/or overlay HUDs. This screenshot shows an example of a sticker (showing a unity inspector) rendering over my avatar's face camera, which renders with ZWrite off at render queue 5000 (Overlay+1000).
Please move these to a more reasonable render queue such as AlphaTest (2450) or Transparent (3000).