I had a map at the maximum zoom level which was 90% explored. Then suddenly (see below)it was completely empty again, and it did not update again EXEPT in the 10% area that was not explored before.

I expected the explored area to remain explored

But then the explored area became unexplored again (the default yellow/brown background of the map).

I am not quite sure how this issue can be reproduced. It happened to me after I traveled off the area covered by the map to where I estimated the centre of the adjacent map would be. I took an unused map with me, activated it there, and then scaled it up to a full zoom map again. Then I stored both the old and the new map in a chest. After I took both maps from the chest the old map was suddenly empty.

When I traveled back to the area covered by the old map there were no updates to the map (so it remained "empty") exept in the parts That I did not uncover before...

After restarting the game the map was normal again, and the extra part that I revealed while the map was behaving strangely was now also shown as one of the explored areas.