I've been building tunnels in nether between portals and when I go back trough them, some chunks go back to an older state of them once or twice. Like I break the tunnel, then when I go back, specific chunks are like before I broke them. They don't entirely regenerate because first I make 1x2 tunnel, and then continue with the 2x3, but when I go back trough them, the 2x3 go back to the old 1x2 tunnel in some chunks like I didn't break it at all. It is a bit annoying in specific regions like basalt deltas where I can't insta mine everything. (I play on a server hosted on my computer, might be a reason)