[1.16.5] Chunks either not saving or unloading properly. Causing Items in chest to disappear.

Hello,

We're getting a strange problem on our server in a private modpack we've created. Players are reporting items are disappearing from chest when leaving chunks, relogging or even dying. Some even reported the items disappeared as soon as they closed a chest. Some furnaces stop working in certain locations and has to be moved to be functional again. Graves from the "Corail Tombstone" mod either appears or disappears and sometimes when retrieving lost items upon death from the grave. No loot is given back. Some users even reported some ore blocks are reappearing in Caves after mining them.

Only clue I've gotten is from the logs where the console is spamming various chunk related errors, "Failed to save chunk" or "Except during firing event". Using the "Save-all" command reports that the world has been saved successfully but the issue persist. Any ideas? 

Here is a picture taken from a user whom inserted new items into a chest at the bottom. They left the area, mined for a few minutes. Returned to the same chest and the items are gone. 

2021-03-28-1.log.gz