merging lines in wrong point order bug

Sometimes (I did not find a rule) merging two lines causes the system to delete the rightmost point of the rightmost line and to move it to the left point of that same segment, so that this point is in fact doubled, one exactly over the other. This is impossible to see if one does not grab the point and move it away. It is no doubt the bug that is the most timeconsuming in segmentation correction on the user side. If I have 10 lines on one page and one of them is merged with this bug, correcting the bug takes me more than twice the time than merging the other 9 lines. We cannot allow this bug to survive this week. Please kill it. This is what I mean: