Model and texture not showing up, doesn't save one of the assets.

[rovenfaded]

So I'm having a few problems and I'm not sure if they're related or not.

I spent a good few hours modelling this:
model.zip
I put it into a resource pack along with all the necessary assets and stuff and... nothing. I wanted to test it, so I had it replace the diamond_block, but in game the diamond block just shows up as a regular block with missing textures. Neither the model or the textures seem to work.

So I tried making two quick small models. Both worked perfectly. When I opened the model save file, I noticed something wrong. It deleted one of the textures from the texture manager (the gold_block). Yes it was there before. I had applied it to the model. So I did that again. Saved the project, close the program, open the program, load the project. Once again, no gold_block texture and it was removed from the texture manager.

I can't figure out why this is happening to this one specific model. Like I said, the other two smaller ones worked perfectly. I've spent multiple hours into this model and I would hate for it to be wasted.

EDIT: Okay, so I tested a little, and after changing the gold to yellow concrete, it was able to save the project successfully. So there is either a 14 block texture limit that isn't mentioned anywhere, or gold blocks are just impossible to use. The model unfortunately STILL does not seem to work. Is there maybe a limit to the amount of objects that isn't mentioned anywhere?

EDIT2: Tested a bit more with some simpler models. Took one that stretched out two or three blocks. Turns out that's not possible. I'm noticing that there are a few things that aren't specifically mentioned that might be handy to know before wasting 8 hours on a model that will never work.