Just to add a little more information:
This does not happen on vyos-1.5-stream-2025-Q1 (although this release also has issues regarding the state policy: https://forum.vyos.io/t/have-to-delete-firewall-global-options-state-policy-invalid-after-upgrading-to-1-5-stream-2025-q1/16131/7)

Yes, as described in the issue, using dynamically added routes works fine. But I want to add the routes manually as I did before with the 1.3.x release.
Using both automatically added routes from PPPoE and setting them statically does not sound like a "sane" config to me.