Quick addendum – we’ve done a lot of experimentation on this as I like to build a strong bug report when I report problems like this (see Microsoft Store and Ultimate Edition issues from a few months back), but in this case, outside of some buboes not properly “aciding”, there is a factor preventing this achievement from popping that I cannot suss out via experimentation (at least not yet). Several “perfect” runs have led to no achievement unlocks, and there is no connecting issue we can ascertain.
If I do find something, I’ll report it here. Hopefully we’ll finish the remaining DLC achievements next week and can look into this again. As of now, we have spent WAY too much time trying to figure this mystery out.