Responded to @kasparsd:

Turns out this happens due to a stale tracked connection that RouterOS has in the firewall which points to the expired passthrough DHCP IP. Seems like a #RouterOS bug.
Responded to @kasparsd:
Turns out this happens due to a stale tracked connection that RouterOS has in the firewall which points to the expired passthrough DHCP IP. Seems like a #RouterOS bug.