The dashes next to the semi-colons seem to confuse systemd so drop them.
[Service]
ExecStart=@PLYMOUTH_DAEMON_DIR@/plymouthd --mode=boot --pid-file=@plymouthruntimedir@/pid --attach-to-session
-ExecStartPost=-/bin/udevadm settle --timeout=30 --exit-if-exists=/sys/class/drm/card0/dev;-/bin/udevadm settle --timeout=30 --exit-if-exists=/sys/class/graphics/fb0/dev; -@PLYMOUTH_CLIENT_DIR@/plymouth show-splash
+ExecStartPost=-/bin/udevadm settle --timeout=30 --exit-if-exists=/sys/class/drm/card0/dev ; /bin/udevadm settle --timeout=30 --exit-if-exists=/sys/class/graphics/fb0/dev ; @PLYMOUTH_CLIENT_DIR@/plymouth show-splash
Type=forking
KillMode=none
SendSIGKILL=no