https://pastebin.com/uS1Mt3SA
on version 1.12.2-2.7.0.574

it seems to have been triggered by farms using mailboxes to ship stuff to sorting/processing
4 treefarms, and 8 crop farms, spawn chunk loaded for the origins of the treefarms.

Each mailbox pairing was public and not overlapping or merging into a single box.

Looking at this I am not sure what really happened here. I can see the error but I can't see how simple merging of a stack into an inventory of 18 slots would take that much time and I definitely can't see any goofy loops in the code. So I suspect this was some strange of events happening at the same time.

Or have you got steps that consistently recreate this issue?