I’d guess that the awareness of a network connection is managed by conman and conman doesn’t observe that ethernet interface. In my previous SFOS upgrade process it happened that Silica didn’t start anymore and I could only connect through USB from my desktop. I set up routing on the phone but tools like pkcon didn’t work because they reported that conman said it had no internet connection.
So I’d suggest to figure out how to tell conman to observe your USB-Ethernet interface.