PPPoE DSL broken since 6.0

In 5.x this worked for a user:

nmcli connection edit type pppoe con-name "DSL"
set pppoe.username
save
Y (yes)
set pppoe.password
save
quit

… and reboot.

In 6.x this fails with "Connection failed. Activation of network connection failed".

This might have the same root cause as #20291 (closed).

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