The resource pack attached offers a complete fix for this issue. Mojang have my full permission to (and are encouraged to) replace the affected vanilla model files with the contents of the resource pack.

Download resource pack:

• 21w39a: consistent-hollow-blocks-21w39a-v1.0.zip

The bug

The leaves block model, which is functionally a template model due to only containing the block shape and being referenced by other models to apply textures, is not prefixed with "template_". This is unlike a multitude of other template models which the game uses, which are in fact prefixed with "template_".

How to reproduce

This is as easy as just navigating to the assets/minecraft/models/block directory and checking the file names.

1. Open the game jar file using one's preferred method
2. Check the model file of a block that uses the leaves model, such as oak_leaves.json
   1. Note that it references block/leaves.json, and also note that it isn't prefixed with template_
3. Check the model of a block such as a wall torch, anvil, chorus flower, ...
   1. Note that the model it references is prefixed with template_ due to the referenced model in question being a template model

Expected and actual results

The table below summarizes the expected file names for the models in question.

How to fix

Fixing this is as simple as renaming leaves.json to template_leaves.json, and then repointing all model files that reference it to the new names. 6 different model files currently point to it, matching the 6 leaves types that use tints (azaleas do not).

The resource pack attached to this issue fixes this issue in a different (and I'd say better) way - rather than just rename leaves.json to template_leaves.json, it goes ahead and redefines hollow block models: powder snow's model has been changed to a template model, which powder snow is made to point to, as are spawners (fixing MC-169969). A tinted version of this model is also implemented, which the six usual leaves point to (fixing MC-171859 - the azalea leaves are pointed to the non-tinted model instead due to not using tints). As such, this issue can be declared fixed as the leaves template model now has template_ at the start, although not being called template_leaves.json but rather the more general template_tinted_cube_hollow_all.json. These new models can be added to minecraft.jar in seconds to fix this issue.

The pack also cleans up formatting for consistency.