I think they said before release that things like this might happen.
They also said that they are working to fix it.
Indeed, it is much better now, but oh boy, that was all over the place for me before the fixes.
Much less now, but it still happens. And sometimes the algorithm that determines which parts of the geometry aren't being looked at bugs out. Sometimes geometry that wasn't fully out of the screen would have its textures unloaded, especially if it's far away and at the edge of the screen, even after the fixes.