I think I finally have a clue what’s going on here, and a workaround that has so far fixed the problem for me in several test iterations.
The WorldDataLayers Actor in the affected levels was throwing a warning in the Message Log indicating “Actor needs to be resaved”. I thought I was taking care of that by doing a Save or Save All after seeing the warning, but it seems that was not the case.
The workaround is to right-click on any WorldDataLayers Actor in the Outliner and choose Save Selected Actor(s) from the context menu. After that, I am able to rebuild the Mini Map without crashing.
So far this method has worked in every level I’ve tested.
There is still something strange going on with respect to WP in 5.1, however. In one level all the LandscapeStreamingProxy Actors were greyed out as if unloaded, but the landscape was all clearly visible, and when I entered PIE mode the editor crashed with a memory access violation. I sent the crash report to Epic for that. It may be that something was corrupted on that specific level, because I have not reproduced that behavior on the other levels. Perhaps during one of my earlier crashes the Actor files were corrupted somehow.
I would appreciate reports from anyone else who has been able to reproduce the bug described in my OP, and see if this workaround is also effective for you.