Been an issue for a long while which feels very much ignored and happening on multiple gaming devices, for me it's on the Xbox One. Large portions of discovered map completely disappears either after logging out of the game and back in, or when you go in and out of a volcano. It ends up looking like a massive line down the map. Sometimes re-logging will just change the area which disappears, so the missing locations have to still be in the save files, just not loading up into the game. This only seems to happen after discovering large areas of the map, days playing don't seem to affect it. Unfortunately can't get my picture to upload on this site.
Hoping the issue isn't going to be ignored as I've seen this complaint made on various other sides dating from a few years ago. Is it still an ongoing problem for others or am I just unlucky?
Best way to cause the issue is to discover as much of the map as possible, then log out completely and back in. Or go inside the volcano.