Paintstormstudio.com

Mirroring with transform tools has problems with flipping/mirroring.

Grab side handle and move it to other side.
It should just flip but often it goes in opposite direction.

Making few circular motions usually helps to fix it.

PS.
I hope this is a place for posting bug reports if there is dedicated bug tracker let me know and sorry but I couldn't find one.

Yes, we know about this bug. Better works if you will drag by the corner point and do it not very slow. We'll fix this bug as soon as we can.