Roblox has problems, but it’s not an utter failure. If you hate it so much, move on and quit Roblox development don’t keep doing what is bad for you. Programs are going to have glitches/bugs no matter the program being used. You could always post a bug report on the forums if you think it is a glitch/bug. That would be in category #platform-feedback:engine-bugs
I put a generic decal on a Union and moved it from the Workspace to a model, very similar to what you showed in your gif. It really is an odd issue, and I’d like to help you as much as I can.
Can you provide more info on any scripts, other objects, etc, that are in the Unions, or in the model? I will also add that Unions themselves are really nasty to work with in all manners on development, and it could just be another case of “Unions suck.”
Well, like I said before, I can place the decal on every other part in the group except for the 2 parts I want to put the decal on. I was able to put 2 decals on the tail, which was also under the same group, and that had no problems.