Hi Petr, thanks for reporting it, it looks like it has to do with the order the objects are being processed in.

It looks like when trying to union together the 2 "outside" circles that do not intersect first, it is getting confused at that point and not making the proper result, but only when the 2 non-intersecting objects have reversed directions from one another like you will get with a mirror.

When you do 2 intersecting ones manually, or edit the objects in such a way that their internal ordering in the geometry database has changed, then that can make it work since it will process 2 intersecting ones at first instead of 2 non-intersecting ones.

I'll dig into it a bit to see what is involved in fixing it up.

- Michael