dag-ed
If a material contains the VRCFallback tag and the fallback type is not set to "Particle", then the VRChat/Mobile/Particle/(Multiply|Additive) shader on Quest will be forcibly replaced with a white, opaque, single-sided shader.
* This issue does not affect PC builds.
* it AFFECTS older avatars that contain the same type of materials.
Steps to Reproduce:
  1. Create a new material and assign the Poiyomi Toon shader.
  2. Set the VRCFallback shader to Toon/Fade/DoubleSided.
  3. Click "Lock in Optimized Shader".
  4. Change the shader to VRChat/Mobile/Particle/Multiply.
  5. Assign a texture to the material.
  6. Apply this material to a particle renderer.
  7. Add the particle system to an avatar.
  8. Build the avatar for Android.
⇒ On Quest, the particle will render as a single-sided white billboard with front-face culling and no visible texture.
note: This issue is reproducible with any shader that allows setting a VRCFallback tag and fallback type.