Boolean subtraction errors

[EricTheFish]

I'm using Moho12.5. Doing a simple Boolean subtraction, for example a circle with a rectangle overlapping one side intending to cut a chunk out of the circle.

After welding crossings and removing unwanted lines, when the last unwanted line of the original circle is deleted, the remaining circular line distorts. This is frustrating.

Is there a workaround? I've just installed Moho13 Trial and it behaves the same.

thanks EricTheFish