I found one chunk that previously existed and now it has disappeared through "not loading anymore"
since i play minecraft more than 4 years, i know it happens sometimes and i wanted to reload and restart the game. In SP it shouldn't happen.
After reloading/restarting map: no effect, still chunk unloaded
Restarting the game still didn't change it

when i last stepped into it, i was buging around in the upper layer of the territory, only suicide could save me

So there must be another problem. i hope you can fix it