well its not really a bug, just really stupid programming.
I had an offer with 4 lots. 3 of them sold the last one didn't. When I deleted the unsold lot from the trade-history, the 3 other lots from the trade disappeared as well. I didn't want to delete those, I didn't click the delete-X for those. But I guess they were linked to the other lot from the trade. Hmm. That's.... silly.