When adding a block tag to FlammableBlockRegistry, it is ignored, which means blocks in this tag cannot be flammable. If you /reload, it can be normal, even if you re-enter the world. But when you re-launch the game, it will happen again.

minecraft_version=1.18.1
yarn_mappings=1.18.1+build.22
loader_version=0.13.3
fabric_version=0.46.6+1.18

Can't reproduce in 1.19. Please ensure that you are registering the tag early enough (i.e. in your mod initializer), and let me know if the issue persists.

Looking at the code, it seems that it should be an issue though... I am a bit confused. 😅

This is indeed an issue, I think I was unable to reproduce this on Fabric because of the experimental settings warning causing the resources to be loaded a second time... The simplest fix would require #2265 to be merged, so I could use the TagsLoaded event.