I played with my daughter in our Minecraft world. My daughter built a nether portal and went into the nether. She went exploring near the Netherspawn. I got a few more things from our house. When I was on my way to the portal, my daughter said the portal was completely gone. Since we're playing in split screen, I was able to see it for myself. It was completely gone. So I got obsidian from the house and went through the portal afterwards. But I haven't arrived. Both arrived and exited the world spawn. Portal in the Overworld was still activated. When I re-entered alone, it was the nether spawn my daughter had too.
Got the same problem in my world. Actually 1-2 times a day. Copied the world and save often. The problem with saving is not always, sometimes it works and sometimes not. But I can say that the memory problem occurs very often when I play on the PS4 with my daughter. Saving only worked once. I noticed that if the save didn't work, the chunks weren't loaded beforehand or only some chunks....