Disappearing Structure

Took a screenshot of a structure with coordinates for me to explore later (as my inventory was full), came back only to see that most of the structure (in this case, a ruined overworld portal) is gone.

 

(Additional note: I did not get up close to the structure prior to the "later exploration", as it can be seen in the screenshot. I just took the first screenshot while exploring in the rivers far away and I was too lazy to get up close to the ruined portal)

 

(One is a screenshot I took 2 days before going back there, one when I finally went to the place again)