UDP Source Port rule doesn't match at first.

[kathampy]

WAN1 is the default gateway.

Rule 1
Protocol: UDP
Destination Port: 27000-27030
Gateway: WAN2
Action: Allow

Rule 2
Protocol: UDP
Source Port: 27005
Gateway: WAN2
Action: Allow

Rule 3
Protocol: Any
Action: Allow

Whenever I launch the application which is configured to use source port 27005, it initially uses WAN1 and not WAN2 as specified in Rule 2. Rule 1 which uses the destination port does work. If I disconnect WAN1, then it immediately starts using WAN2 and continues to do so even if I reconnect WAN1.

The reason it suddenly starts working when WAN1 is disconnected cannot be because of the automatically recreated rule when the gateway is down, because the default gateway is WAN1 not WAN2.
There are no states in the firewall for UDP port 27005 at all, even when data is actively being transferred on either WAN1 or WAN2.