Non-wiz antags not triggering properly on mixed rounds

[Roomba]

Twice now, my name has ended up showing up as an antag at the end of a mixed round, despite not getting an antag popup at the start (but no regular objectives). Checked my notes and all, nothing. As far as I can tell, the only other similarity was that there was a wizard involved in both rounds, who activated just fine. Not much of a link, but  I don't know what else could be causing it.