Good catch!  You don't even need to wait for a reload - if you flatten a tie (without nudging it) such that it collides with a staff line, it actually gets laid out again and snaps away from the staff line as soon as you leave Edit mode.  If you then drag it back, it stays, because I do check isNudged().  Only non-automatically-adjusted ties that are edited but not nudged will show this anomaly.

I can fix this it by also checking to see if any of the grips have non-zero user offsets.  Or we could call it a feature - you have to try extra hard to make a tie collide with a staff line, so it's kind of like a confirmation dialog, but you can still do it :-).

At one point while working on this, I added a "SlurSegment::isEdited()" function that checked all the grips.  This is something I was thinking should probably exist at the Element level, but no need to deal with that now.  I'll add another commit to add back the isEdited() function and check it in addition to isNudged().  That should fix this case.