Routing failure on a multihomed Devuan laptop when ethernet cable removed and wifi connection active

I have a Devuan Chimaera laptop (GNU/Linux, based off Debian) that has ethernet and wifi connectivity. When I connect to the wifi and take out the ethernet cable, I expect packets to no longer be routed out the ethernet port, and to go via wifi (which works fine when used alone).

This isn’t what happens – in order to ping 1.1.1.1, I have to bind ping to the wifi adapter – checking ‘ip route list’ shows the following:

default via 192.168.188.1 dev wlxb8b7f1d4cb8f proto dhcp metric 600
192.168.188.0/24 dev eth0 proto kernel scope link src 192.168.188.33 linkdown
192.168.188.0/24 dev wlxb8b7f1d4cb8f proto kernel scope link src 192.168.188.34 metric 600

When I delete the default route over eth0 everything works. The kernel knows the link is down, so why is it routing packets over the dead interface?

For reference I’m using network-manager-gnome 1.20.0-3.