If you union a part with a mesh that has an offset, the offset will be ignored and the final shape of the union (if it succeeds) will be what it would be if there were no offset. I’ve attached a file to show this; unioning the two Parts will produce the union in the middle. This problem also occurs with Cylinder and Sphere SpecialMeshes, and when you Negate parts.
Side note: do you notice anything unusual if you drag both spheres?
Haven’t tried Unioning Offset Mesh Parts, but I have noticed (about a week or more ago) that dragging or moving an offset Mesh cancels the Offset until the Part has stopped moving.
Kinda strange but it hasn’t caused me any issues so far.
Bump. CSGs take mesh Scale into account, so I’d expect them to take Offset into account as well. OP’s repro file was lost during the forum switch, but here are simple repro steps:
Insert Part from Advanced Objects
Insert BlockMesh (or any type of mesh) into the Part
Result: Union ignored Offset
I still have the original repro file and I have attached it to this post.
CSGOffSetError.rbxmx (24.8 KB)