Это поведение Network Manager, начиная с версии 1.8, см. коммит:https://github.com/NetworkManager/NetworkManager/commit/6b7e9f9b225e81d365fd95901a88a7bc59c1eb39
Там написано:
This makes it possible to retain Internet connectivity when multiple devices have a default route, but one with the link type of a higher priority can not reach the Internet.