If u have mirrowed objects the brush rotation (movetool) is also mirrowed on the mirrowed object even if u apply the mirrow repeater.
Thants annoying 
That’s not a bug.
And maybe show what you are doing because “validating” a mirror can mean 3 thing (instance, uninstance or join).
Ah ok, validating has no effect to it, but if I join and seperate the mirrowed objects the mirrowed rotation disappeared.
thanx