I have every confidence in your Mike to be able to correct this bug. And I and I imagine everyone else here is very grateful for the hard work you and effort you, @Kier and @Ashley put into developing, producing, and releasing XenForo.This is likely going to be a major pain/issue to fix, as it doesn't apply across the board. It only applies to attachments in conversations where the deleted user is the only person left.
Flipping this around, it's also possible for it to come up when uninstalling an add-on if the add-on allows attachments.
If you need someone to help test a fix before release, I'd be very happy to lend a hand.