I can't pinpoint the exact reason why it is happening, nor do I have screenshots of it, but at times, some items go randomly missing from my "View Your Listings page". Then a few minutes later, they will be back....
Similarly sometimes an item that is listed on the item market, will not reflect on the API. like the API doesn't show it under my listings, but then later the item pops back up. (I'm not sure whether the item goes missing together on both view listings and API or if they r separate instances.)
I have a script to record my listings and sales using API to semi-automate the process. Yesterday, the Red guandao rage was missing from the API under my listings and it didn't show on the view your listings page either... Few hours later, they showed up again.
Right now, an M4A1 Colt Carbine "M4A1 Colt Carbine Quality: 180.65% Disarm 6 nan nan. Listed Price: 300000000" has gone missing from the view your listings page and the API user -> itemmarket['listings'] and it'll probably pop back up in a few mins/ hours?