If you offer a scratchcard and then scratch said card this will happen but there are other ways to trigger it too, like one party switching languages, there's certainly other causes but I do not know them.
This happened to me when one of my trades timed out. I put it in my SDB, went to sell it a couple weeks later and it was gone. I submitted a ticket and they did give me a replacement. I have premium, not sure if that helped?
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