Just got back to this after 3 weeks. All of a sudden this does not work any longer 
The following rule
3 chain=output action=mark-routing new-routing-mark=default_wwan
passthrough=no dst-address=192.0.2.1 protocol=udp dst-port=51522 log=no log-prefix=""
does not match any longer (packet counter stays at zero), even though I mark the packets with route marking “default_wwan” in the output chain.
Why the heck could this not work any longer?