Hi Marcel ,
Actually you might be having an issue with your existing VPN network connection still having ipv4.ignore-auto-routes enabled, see: https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/946#note_1350911 my previous suggestion of switching from strongswan to libreswan, may or may not help. I wasn't able to see the strongswan issues with network-manager >= 1.36 on Debian I saw with Arch Linux, but doesn't mean others wouldn't. Cheers, Doug