Hi,
I also have this problem. Zones (including the world map) show up now as non-explored and I cannot change party members.
For me, the problem started after I finished the Old City area and returned to Dereo. I noticed the Denver's Row showing as unexplored, with all doors closed and containers unlooted. I likewise did not pay this much heed, just assumed it was a minor thing. I didn't notice any other hints of something being wrong until (quite soon after) when I returned to the palace and after having finished the Gullet questline, I wanted to swap away Tekehu to Xoti. It was then that I really noticed the bug happening.
I can't swap in any characters that were not in my party earlier. If I try, once I close the party management window, I just have empty slots in my party. I can't add in any characters to fill that slot except Tekehu, who was in my party when the bug apparently started.
Tbh, I did not immediately even connect the party management bug to the "map reset" bug. It was only when I decided to see if I coulld swap party from my ship that I realized that all the areas I had explored out at sea were under the fog again and this made me think they might be connected.
I do have a save from before I went to the Old City and in that one it seems like party management works. I have some saves left in Old City as well, so I could maybe try to triangulate a point where it breaks and see if it does so again. Though I have to say this whole this has kinda soured me a bit for the game. I don't much enjoy having to backtrack, especially since I struggled through some fights in Old City and not too keen on having to redo it. Shame, because I too was really enjoying the game, even more so than the first one.
Btw, I didn't install any patches or addons recently to my recollection.