I play on the latest edition of Bedrock Edition Minecraft (1.16), and I have been playing on a Hard Mode Survival world in Singleplayer for about a month now. Recently, I had made a tunnel that was approximately one thousand blocks with numerous pickaxes. I didn't go back down into the tunnel for a while, since I had other projects to do, but it had been completely dug through in a 2x1 hall.
But, when I went back down to the tunnel, I noticed that part of it which had formerly been dug out was now back to stone. When I ran to the other side of the tunnel and went down through the entrance, it seemed to be fine for a while before I reached the same issue; more stone. I didn't bother digging back through, and I do believe it went away (I haven't gone back through the tunnel completely but when I ran through to get to a cave, there wasn't any blockages). I've had this issue in Console Edition, but I always thought it was something to do with multiplayer messing with the chunks saving. It'd make more sense for all the chunks of the tunnel to reset and not random ones.
Despite this, I am still curious, what causes progress in chunks to reset without forcing a chunk reset in a cheat-disabled survival world? Bonus points for solutions that can fix this.