I have already submitted an in-game bug report on this issue. However, that method does not allow for a response concerning the problem.
The missing items are replaceable. The problem is that with four tabs filled, I do not know which of the items are missing. I just find 1-2 open slots repeatedly.
I do know the first two that disappeared - Stolen Shado-Pan Insignias (over 100) and Celestial Insignias (number unknown). During the event I had made five tailor toons and wanted to advance them to “exalted” with the August Celestials. I was disappointed to find those items missing. I spent many early morning hours collecting replacements and was able to fulfill the requirements for the reputation on all five of the toons.
The other missing items were classic mats and are easily gathered by my toons.
Since this is a reoccurring problem, I have cleared all the items from the Warband Banks to ensure that other items do not go missing. I will use the Warband Banks in the future to transfer items immediately from one toon to another toon. However, I will not store items in the bank long term until I know that this bug has been fixed.
I regret spending the large sum of gold to purchase those tabs, but gold is also replaceable - it will just take some time and effort.
I like the idea behind the Warband Bank - I just wish that it worked for me.
Update: I ran a test to see if items still end up missing. I put three items into the bank a few days ago and only 2 remain in the bank as of October 14, 2024. I hope this problem will be solved (or am I able to get my gold back for purchasing those tabs). I could find other ways to spend my gold.