Just to add to the "picture" if i disable the gateway on the WAN interface everything starts to flow through OPT1. One additional line is logged in the system log in this case that does not appear if i don't disable the failing gateway:
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May 10 18:33:21 opnsense: /usr/local/etc/rc.filter_configure: GATEWAYS: Group WANGWGROUP did not have any gateways up on tier 1!