I just found out that when using Route Based IPsec tunnels, Port Forwarding only works if
Firewall / Settings / Advanced / Use shared forwarding
is disabled. It is enabled by default. I wonder if this is a bug or spmething that is specific to my environment. If this is by design, it should be added to the manual as it is not obvious.
This was obsered on a fresh installation of OPNsense 19.7.9.