I try not to use G41/42 in Mach if possible as it seems a little unpredictable . . often switching sides in the middle of the toolpath . . usually at long arcs . . . .
I use comp for just almost everything I cut, and have never seen it switch sides. But I have seen a bug where the tool just takes off out into space, but it wasn't my part, and it was a profile made up of many short segments, which comp doesn't like.
The message seems to change depending of if the lead-in is a straight line, an arc,
According to the test version Brian posted a few years ago previewing the V4 comp, leadins will no longer be allowed to be arcs. So you might want to stop using that practice now.
Mach cutter comp is supposed to work as described by Smid in his programming books.
I don't have the book, but I know that comp works the way I use it.