So, really obscure bug... Basically, if you remove all references to fire_layer_1 from the model files, the "on fire" texture appears as a missing texture. The reason is likely since the game is no longer adding that texture to the atlas, so when the fire overlay tries calling it, it comes up blank.

Why would someone remove references to it?

The reason someone would remove references to fire_layer_1 in favor of a custom texture are as follows:

• Since that file is used for the fire overlay, replacing its usage with another texture allows the block to be independently textured from the overlay.
• If the resource pack uses only one fire texture, the author would not want to use two separate files, for the sake of efficiency

This is obviously a broken resource pack

No, it is not. I know resource packs and block models very well and have checked for errors, and none exist. This is a Minecraft error

Also, all resource pack errors are always printed in the console log, and none are printed.

Solution

This can be be worked around by adding the texture to an unused "particle" tag, but really those should not be loading textures anyways, since the game never uses it.

To actually fix this, the texture should ignore whether it exists in a model file, and load anyways. Or, a more ideal solution would be the "on fire" texture always using the file from the "particle" tag of fire.

Steps to reproduce

1. Remove all references to fire_layer_1 from the block models in a resource pack, or use the resource pack download provided.
2. Step into lava or fire (will have to be lava in creative mode)
3. Notice the missing texture