The blocks turned into invisible both model and game ones, and here’s the way i’ve got to reproduce it:
In short, the normal blocks from game were completely functional, and also the models apparently, but when i switched to 2nd and 3rd layers, the blocks got invisible.
Then i deleted the model, and created another one, so you can realize that the normal block had a way to be placed but didn’t had a way to be deleted, even when it no more exists according to game, but remained its visual shape as you see.
And to finish, when i’ve summoned once more the model, basically every single layer was invisible, and for now also all the normal game blocks turned into invisible.