The issue seems to be a misconfiguration in the standard
/lib/systemd/system/ptp4l@.service: apparently, it starts already when the
network (hardware) is not fully up and running – at least on my Raspberry Pi5
is not sufficiently up (running current Debian bookworm from Raspberry with
Network Manager (not(!) dhcpd-base), as is delivered in the standard image).
Instead of just
After=sys-subsystem-net-devices-%i.device
it requires
After=network-online.target
Wants=network-online.target
Thank you!
P.S.: Hat-tip to @jclark @
https://github.com/raspberrypi/linux/issues/5881#issuecomment-2567484885
