Plugin assumes /run is available (and ebuild orders to create it), but doesn't even try to create a "plymouth" sub-directory on runtime. It is necessary, as udev when detecting /run automagically mounts it as tmpfs overwriting leaving only "lock" and "udev" in /run.
Please fix it - either by creating missing directory or by not hard-compiling pid-file path into plugin.
Creating /run/plymouth if not present. Fixes #1.
Thank you for your report!