I had actually already fixed this - it was a bug in the earlier betas with the conversation rebuild (which needs to be run to add the recipient list to old conversations). We just hadn't done the rebuild since the fix.
I have, however, tweaked this code again. Previously, it just hid the fact that there was a deleted member in the conversation. It will now list "unknown member" as that's really all we know (similar to the list in the conversation).
Actually in the conversation view, I do see the users username. So I can identify the user by the username. Now I see in conversation_list "unknown member" and in conversation_view "the_former_username".