It was an IPsec VPN!
If the near and far networks overlap then traffic heading for the firewall IP needs to bypass the VPN.
That's normally sorted out by the 'Enable bypass for LAN interface IP' setting with is on by default.
However, that only handles the lan interface and not any other lan-type interfaces which get created.
The fix was to add an 'Additional IPsec bypass' rule.
I'm not sure if this is a bug or not.
Should there be a list of interfaces to bypass rather than just the lan interface being special?
Cheers,
Scott