[Xymon] Acked alerts occasionally continuing to email?

Running xymon 4.3.17on rhel6

We are occasionally seeing custom test yellow alerts continue to email after being ack'ed. It's been hard to pin down, because if I send a green and then let them go yellow again I am not seeing the problem recur. They appear as acked in the web interface.

Wondering if anyone else has seen anything like this?

(major boss-annoyance factor here)

thanks Betsy