The bug

These two template models, used for spawners and trial spawners, use a negative-size cube for rendering inner graphics, which is unlike all other hollow cuboids in model files, which use six planes instead for the same effect.

This causes MC-266463, as while the cullface arguments are specified correctly, negative element sizes do not handle them correctly.

Using negative size cuboids appears to be discouraged, due to a change in 24w38a in which redstone torch models were changed to use the six planes approach:

Comparison of the model file:
Before:

After:

How to fix

Make these templates more like the powder snow model. I've attached a resource pack that does just this, fixing MC-266463 and MC-266536 in the process. Mojang have my full permission to use these resources to fix this issue if desired.