Mountpoints are explicitly/hardcoded to never be umountable and removable via tmpfiles.d specification.[1] Possibly because such mountpoints are generally expected to be systemd .mount units, and as such unmounting and removal of paths would be tied in with systemd unit handling, rather than systemd tmpfiles.d handling? I suppose one could still create tmpfiles.d configuration file that handles files within a fixed mountpoint of /mnt/alwaysmounted/tmp/ as I couldn't see anything in tmpfiles.c[1] at a quick glance that would prevent this occurring.
