If you try to load any .schematic file with litematica it will be empty (I assume because blocks aren't being upgraded from legacy to modern format). Disabling Dynamic DFU fixes it

It is probably caused by this Litematica mixin: https://github.com/maruohon/litematica/blob/pre-rewrite/fabric/1.20.x/src/main/java/fi/dy/masa/litematica/mixin/MixinBlockStateFlattening.java

In vanilla, that class is indirectly loaded at startup by DFU being initialized, and so the mixin's injector will run. With dynamic DFU enabled, it only loads when DFU is first invoked to upgrade a chunk - but Litematica probably doesn't use DFU directly, since it seems to be adding these entries to its own conversion map.

I'll just disable dynamic DFU automatically when Litematica is present.

Should be fixed in 5.12.0.