Thank you!
It’s still a bug, just harder to see how it’s reproduced. A bug perhaps in converting a 4.16 project .umap file to a 4.17 file.
So, what I did was select everything in the map and copy it. Then create a new empty level and load that level. Paste everything into the level. Finally remove the old level and rename the new level to the old name.
There was still a glitch (one object lost its references), but I deleted the object and re-added it to fix it. But the save issue no longer happens.