Outbound NAT with port forwarding weirdness

[Brano]

System version: OPNsense 21.1.5-amd64

I have a simple port forward defied with http port on wan side redirected to local server.

When outbound NAT is set to Automatic or Hybrid the port forwarding works fine.
When I manually configure outbound NAT as shown on the picture, port forwarding also works fine.

However, when I change outbound NAT source to ANY (instead of defined RFC1918 subnets), port forwarding stops working! Why is that?

Thank you.