I have recently made some blocks with custom 3d model. The models of these blocks were made using Blockbench and the transparency of the blocks was set to cutout/translucent (I tried both).
The blocks do not cause any recompilation error, but they have missing texture when placed and a weird texture when held in hand.
I made custom blocks in different workspaces, and they have the same bug (blocks in Pyromancy were supposed to have a custom 3d model).
These are not the first custom 3d blocks I make in 1.8.0 (previously, blocks made in 1.8.0 worked).