Just a theory…has it been confirmed the bag was actually checked in to the hold? Any possibility that when the crew said it wouldn’t fit in to the overhead bin, they put in the wardrobe at the front of the aircraft?

I could see that due to some miscommunication, the OP misunderstood this to mean gate checking and proceeded to baggage reclaim upon arrival not realising their bag was still onboard. Which is how it came to be treated as lost property, rather than luggage “in the system.”

Hopefully the OP can come back and provide further details soon.