If you unionize concentric rings that have the same height, the inner rings disappear:
This does not occur for non-concentric rings of the same height, and does not occur for concentric rings with different heights.
How to recreate: create two rings with the same height and different diameters. Move them so that they are concentric (they have the same CFrame) and unionize. The inner rings will disappear
They can’t be unioned if they are not connected. The way that the faces are stored probably required that all faces are connected through other faces to all other faces.
Edit: just re-read that
Are you saying that if you move the middle ring up, the union works properly?
yes. You can rotate or translate the inner or outer ring in any way and the mesh will compute properly