Ran a normal LDoN this morning, and got credited for no kills in a normal slaughter mission (Miragul's, the Spider Den theme). Tried zoning out and in, relogging, though neither fixed the issue. Not sure what might've caused the problem, since I didn't do anything different than I normally do for an LDoN run.
Usually, I would suspect this was some kind of a timer/lockout issue, but it was only the second LDoN I'd done today, and I'm not using my main (hooray, double exp!), so I'm not clearing the instances terribly quickly.