When a trigger is acknowledged, the trigger will show acknowledged in the dashboard and Events tab.
However, if the zabbix-server restarts/reloads, or if the monitored server go unavailable for a short time, all triggers get UNKNOWN status, and new triggers for the same item show up as un-acknowledged again.
This appears to be the normal logic, but as hosts reboot, zabbix-server reboots, or otherwise gets an unknown data inserted, you must re-acknowledge all the triggers again.
However, e-mail alerts do not get resent when the triggers gets reset, as the trigger still has acknowledged applied to it, so any actions that check for acknowledged=yes will not fire.
Given that, the triggers with unknown should not show as un-acknowledged again.

Actions also lose their association with the trigger. Normally, it would show you how many actions were performed for the trigger, but is an unknown value occurs, but then goes back to TRUE (problem), it no longer shows that any actions were sent, regardless of acknowledged status.