We have solved the issue and I have updated the description to reflect that.
The issue is not a bug may still require looking into in order to prevent it from reoccurring (as well as preventing more bug reports on the same issue).
No, I do not have the information from my player's end. However I did notice that this still occurs in 1.13 and I have attempted to update this report.
Something new did occour though, the first time it occured the server said:
[14:47:54] [Server thread/WARN]: handleDisconnection() called twice
Yes, we optimized the world for the new update. However the issue occurs on both 1.12.2 and 1.13 before and after optimization.