In my survival world, I connected my Nether portal in my base to the one near a village for quick access to my trading hall. When I exit my portal (on both sides) back into the overworld, a diagonal strip of chunks 30 blocks away from my portal, disappears. I built another portal further away and this happened again, in the same area, but a larger strip.

This happened many times and the only two ways I found to deal with this bug is to quit the world and load it back in, and to move out of the unloaded chunks render distance. I tested another world with the same portal layout and nothing strange happened. I tried slightly moving the portals around in a copy of the world; the same outcome. I copied the seed and moved the two portals in completely different places from the old locations (50 blocks away and no interference). The outcome was a lot more unloaded chunks (still in diagonal strips), this was in the same location as before and in the opposing direction. I quit the world and all the chunks were back. When I tested it again, almost the whole area was unloaded chunks. I can still see hollow areas underground (caves, ravines), occasional mobs, and beds. I have seen a few of these complaints but in my case, my game never crashed or froze, but a few times it took a little longer to load into the overworld.