I already replied in the GitHub issue linked to above, but I will post here as well.
This issue is due to a bug in Firefox's AVIF decoder, it does not support tiled/grid images.
The tile size warning is a red herring, libavif (which is used by Chrome) has no issue decoding tiles that are not a multiple of 64, and it also also ignores the multiple of 64 recommendation when writing grid images.
Even grid images which use a tile size that is a multiple of 64 will fail to open in Firefox, e.g. animals_00_multilayer_grid_a1lx.avif from the official AVIF test suite.