When a player put an item in and took it back out, the durability was restored..

Alright, I don't have enough information. Which item?

a jackhammer is probably the easiest to test .. break and repair a few times to decrease durability..   then place in bank.  Close GUI.  Open gui.. take out jackhammer with restored durability.

All weapons and tools. Also flamethrower generates 50 low grade when switching in and out of bank. Any news on a fix? Thanks.

It appears that any item placed in the bank with a red durability bar (from being repaired) will have the red removed if the bank is closed and reopened causing items to be able to be repaired infinitely.