I stopped Q'ing the end nodes when I read about not doing it. But then I noticed I was somehow changing some of the roads from yellow to blue when moving stuff around, so I experimented a bit to see why.
If I attach a temporary road segment to the end node when it's flagged, it then lights up as a u-turn permitted. I then delete the temporary segment, drag that end node a bit (like I would if resizing a cul-de-sac end point) and then it changes from yellow to purple. I then attach another temporary road to the end and now the u-turn is showing up as prohibited. So just dragging the node around had the side-effect of changing the behavior of the u-turn permission. So something is changing that u-turn as a side-effect of me moving the end node whether I intend to do so or not.
Almost all of the dead-ends in my area are u-turn prohibited naturally for whatever reason (long before I walked into town, as they say). Only a few are set to allow for u-turns (as still flagged by the validator). So if one or the other is a problem, then Waze staff are going to need to do some massive macro change of them all whenever they get it all sorted out, then we need to go in and just edit the exceptions here and there as applicable.