Good news, or bad news. Not sure which. It happened again just now. If that bug reporting link is still good, I'll post a set of save game files from a couple weeks ago when it was still loading the map correctly, and the last save from last night when I closed the game and uploaded to cloud. The set of save files from last night, of course have the reset map problem when opened. I have the player, world, and map file. Hope that is all you need.
There was nothing remarkable this time. I didn't put it to sleep during the session or anything. Played for a bit, uncovered a chunk of map (I was removing crimson, using the terraformer which lightly uncovers the map in swaths). Saved/quit. Saved the three files to cloud. Opened today, map gone. Tried to download those three files, but map is reset. Whatever happens is either in the save file when it is saved or from the last game session that the save file gets corrupted. That much I'm sure of at this point.
Added notes: First, the file size of both sets of saves is too big for the form. If you would like so see both, please let me know before I go to the trouble of putting it on a cloud storage that you will be able to use it.
Second, this is soooo weird, I did a test just now where I took the 10 day old map file I had on USB (it was my backup backup) and used it with the current save game file. Map loads fine. It's older of course, has less revealed, and the map is out of date until you pass by and it updates. But it loads, and saves just fine. It's only the newer map file that gets corrupted.
One more edit: It looks like this is indeed related somehow to how much of the map is revealed. That's why it always seems to trigger around the same amount of exploration for me. I now have a game save set that can reproduce the issue within one play session. Load the old save, see map, explore a little bit using Zenith, save, exit, re-load, map is reset. Can reload the old save and repeat this over and over. I'll post this set of files to Roboarms DM.