The order of directions used to generate the assets/minecraft/blockstates/chiseled_bookshelf.json file uses an unsorted map which causes overrides to be written to the model in an arbitrary order. As a result, the data generator cannot be used to consistently get an equivalent output as what is bundled within the JAR, even if different orders are functionally the same within the game. I would expect the data generator to give 100% reproducible output.

An example can be seen on Misode's mcmeta repository.

Code analysis (Yarn mappings)

The relevant code appears to be the Map.of call in the BlockStateModelGenerator#registerChiseledBookshelf method. A fix would be to use a list of pairs rather than a map, like what is used for the occupied slots.