When two mountpoints are mounted to the same directory, facter only shows the first one that appears in /etc/mtab file. Exemple: For CentOS 7 in mtab file are two entries for the root directory:
rootfs / rootfs rw 0 0 |
/dev/mapper/vg0-root / xfs rw,relatime,attr2,inode64,noquota 0 0 |
Facter will only show the first one as a mountpoint for "/". |