DNS settings from usb0 cleared when NetworkManager starts
NetworkManager attempting to connect with WiFi and failing means that DNS settings from usb0's DHCP get cleared.
This becomes a problem when NM fails to get IPv4 connectivity on the WiFi interface at all, and the DNS is IPv4. Unless the situation is a distant consequence of WiFi being flaky in general while IPv6 getting configured OK, this should work fine.
Either way, giving the USB network to NetworkManager should both ensure that this particular situation works well, and that any future usb0 interactions fit NM's model and expectations.
At the same time, NetworkManager insists it doesn't want to manage usb0. Probably something to fix in NetworkManager.