I am currently experiencing a bug with the 1.12.2 generator in 2020.2 that makes it almost unusable with locked code. When checking out a remote workspace, I do "Regenerate code and build", and then run the mod, and in Minecraft, I see purple/black textures in the GUI for all of my elements with locked code.
I have done some digging locally, and it seems this is a result of the proper files not being moved to src/main/resources/assets/[MODNAME]/models/item if the element has locked code.
It is worth noting that this bug only seems to affect the item texture in the inventory GUI of Minecraft. If you place the block, the texture and model are perfectly normal and work as intended.
I have tested this on MCreator for macOS and MCreator for Windows, both with fresh installs of MCreator 2020.2 and a fresh remote workspace checkout of the mod.