I have a weird problem, I have an old world I'm building on (saved from 2013 on external hard drive) and last week it worked fine but suddenly I stumbled upon this area where no changes can be made.

Everytime I try to change anything in that zone, when I save and quit and go back, it reverts to the former savestate before I started working on it. I can dig out a whole dungeonroom and make a bridge or anything (it's underground), but as soon as I relog all changes is gone, just a normal stonecave again.

It is as if this area is stuck in time.

When I login, I can see my recent changes for a brief second, before it POOF back to a boring stonecave. It is only this area, I can make change sjust fine around it, but not in that specific place. It's rather big, so can't continue working until I know what is is.
Also sometimes when I relog in and place a new block, some blocks around it changes to the new material I chose before save-quit.

The place is around x 300 z 600, so it can't be spawn area protection (since I can remove them just fine)

Also, sometimes the zone reverts while I'm STILL in the level, I can make a sandstone wall, go around a corner and make another change, then when I head back it's a normal stonecave again. I'm superconfused.

I have no idea what file to attach, as I don't know what the problem is. It doesn't seem to be corrupted chunks, I can see and interact just fine with the zone, and it doesn't make me crash, but all my changes is gone when I save-quit and log back in.