I saw trader Bob in the floor in one game as well. It seems like these are issues in particular saves and a new game solves the problem going forward. I have a feeling it relates to how some last when losing an area can cause things to fall through the ground and the trader falls one block before the listing completes. Unlike with a player, where you will automatically be reset/teleported when this happens, the traders don't seem to do this. Of course, it could be some other cause as it seems to only affect trader Bob.