Executing the command show log pppoe interface <interface> does not yield any logs even when the given interface is a PPPoE interface and contains logging data. Manually executing journalctl -eu ppp@<interface>.service confirms the presence of logging data. The same issue is present with the monitor log op-mode command.

There seems to be an off-by-one issue in the journalctl command call template that causes the passed interface name to be missed and not passed to journalctl when running.