I’ve had the exact same problem two times. The first time it affected a door which was a union, second time(today) it deleted basically half of my build when I published it. It’s so frustrating having to re-build everything from scratch. I have provided before and after photos of the unions when I uploaded them(this is the most recent one).
Before:
https://gyazo.com/cf2446e6f6de6f2f02e3d8ff5212090b
After:
https://gyazo.com/aa8e03ada22edbb4a561c7b5245a646b
This is making me pretty upset considering it has happened more than once and with multiple items which took me a while to make. I hope Roblox can look into this and hopefully provide a solution.