The bug

For particle files with 1.10.0 format version, in the particle_appearance_billboard component, the uv field can be left out. If you do, it defaults to spanning the entire UV of the texture. For particles with 1.21.10 format version, it instead defaults to the top left corner of the texture.

How to reproduce

1. Download and open the attached world
2. Run /particle test:no_uv_old
3. Run /particle test:no_uv_new

Expected behavior
Both particles should be identical: dirt squares. They have identical particle definitions save the format version.

Observed behavior
no_uv_new is instead a single shade of brown from the top-left of the texture.