I don’t have this line in /etc/fstab on an Arch install. Yet I’ll get:
tmpfs tmpfs 3.8G 8.2M 3.7G 1% /tmp
tmpfs tmpfs 760M 76K 760M 1% /run/user/1000
from df -hT
.
Also:
Perhaps you could comment the line out in /etc/fstab, reboot and check if it will get mounted as it should by systemd’s tmp.mount.