How many VLAN bridges does netplan support?
Solution 1:
To answer my own question: I guess I've been struck by this bug from 2019: https://github.com/systemd/systemd/issues/14417.
As a workaround, enabling and starting the systemd-networkd socket seems to mitigate this issue for now:
systemctl enable systemd-networkd.socket
systemctl start systemd-networkd.socket
Initial testing suggests that this is a viable solution that also survives restarts. I'll continue testing this issue and mark this answer as solved for now.