I don’t think it is the amount of pieces, there must be another problem, memory leak with specific parts for example. I reduced the part count by over 2000 pieces with no effect. I can play without problem at volcano area or the frozen north, but at the desert area i can merely play 2 minutes before all detail went away. Only other changes besides update with Tavern and al Merayah is the bazaar sandstone set i purchased recently.
And if the problem starts it affects the whole map, and not only the textures went away but also parts of the geometry as seen here.