I've worked out what the issue is!
This bug occurs if you try to set a Window border size (any size, other than none) for Breeze.
This is Settings under Appearance -> Window Decorations/ Window border size (bottom left).
Setting a border size for Plastik (which has one by default anyway) is OK.
But Breeze (default none) doesn't like you setting any. I set Tiny, as otherwise overlapping window just merge into one another with no dividing line.
I've worked out what the issue is!
This bug occurs if you try to set a Window border size (any size, other than none) for Breeze.
This is Settings under Appearance -> Window Decorations/ Window border size (bottom left).
Setting a border size for Plastik (which has one by default anyway) is OK.
But Breeze (default none) doesn't like you setting any.
I set Tiny, as otherwise overlapping window just merge into one another with no dividing line.