The placed version of all waxed copper vertical slabs has a broken model, while the non-waxed versions are fine. The model is also fine in the inventory for some reason.
I checked whether the waxed copper horizontal slabs were broken when in mixed slab form, and they all seem to be fine.

This happens on Forge and NeoForge but not Fabric.

Non-waxed on the left, waxed on the right. Though you can also see that both look normal in my hotbar.

While the game is initially loading, there are various warnings like this in the log:
Unable to load model: 'minecraft:block/waxed_oxidized_cut_copper_slab' referenced from: minecraft:block/waxed_oxidized_cut_copper_slab: java.io.FileNotFoundException: minecraft:models/block/waxed_oxidized_cut_copper_slab.json
Which possibly suggests it's trying to load a specific waxed model, even though the waxed and unwaxed blocks use the same model? Though idk why the item would be fine then.

Those warnings also appear in the log on Fabric, but the final model is fine there.