Interesting, with ddcutil 2.2.1 it seems powerdevil does indeed fail with SIGTERM but systemd does not restart it.
Process: 664671 ExecStart=/usr/lib/libexec/org_kde_powerdevil (code=killed, signal=TERM) Main PID: 664671 (code=killed, signal=TERM)Although for some reason systemd does not restart it anyway... Weird.