This is the third time I’ve seen this, and this time I’m sure that nothing else happened.
I have numerous mcron jobs configured, many of which run every 15 minutes, and one which runs once a minute. Just now, at 19:22, I noticed the minutely cron job didn’t seem to run, and saw that one every-15-minute job which always logs to mcron.log had last run at 18:00. I restarted mcron, and jobs started running again. There is no timezone confusion either: That every-15-minute job logged its first run at 19:30.