I own a realm with several friends and since 1.16 (and now 1.16.1) have been out, we have had numerous occasions of the realm not backing up or saving progress when logging out (it has worked some of the time but has not worked a lot of the time). We have been playing the game normally, logging out, and then logging in the next day to find that half or more of our progress was not saved (for example I went netherite collecting with a friend several days ago and then when I logged in today I found myself back at the beginning of the tunnel without any of the ancient debris I had collected or any of the tunnels dug). This issue had happened to many members of the realm in different locations (it's not an issue for just me).

Expected Occurrence:

Playing, logging out, and logging back in a few minutes/hours/days later with my progress saved.

Actual Occurrence:

Playing, logging out, and logging back in a few minutes/hours/days later with much of my progress unsaved and gone. 

This issue has been occurring since 1.16 was released and continues to occur in 1.16.1.