The nrpe service for a socket under systemd should have --inetd

startup/default-socket-svc.in

@@ -5,7 +5,7 @@ After=var-run.mount nss-lookup.target network.target local-fs.target time-sync.t
 
 [Service]
 Restart=on-failure
-ExecStart=@sbindir@/nrpe -c @pkgsysconfdir@/nrpe.cfg -f
+ExecStart=@sbindir@/nrpe -c @pkgsysconfdir@/nrpe.cfg --inetd
 KillMode=process
 User=@nrpe_user@
 Group=@nrpe_group@