First of all, unions are never known to not be buggy,
Here is a reply where I stated a lot of negative reasons why Unions/NegativeParts shouldn’t be used, and have further listed some solutions and sources to my opinions and claims.
Anyways, although I am not sure why this happened to you,
Possibilities are:
- It’s already messed up before the Union parts, Separating may not have broken it(If it does break, click ctrl+Z to undo, then try to separate it again, since it may be just a simple bug that suddenly occurred)
- A very minor bug that occurs rarely and has never been found by others
- Typical Roblox Studio visual bugs, I suggest try testing the game out and see if it still looks “cut” when your player is near it, it may also be due to the graphics settings
- Your union had a lot of parts, hence it’s pretty hard to separate it since Roblox Studio Union modeling isn’t known to be efficient at handling complex shapes