This is correct. Unless you mount with sync. See more from man mount.
I usually do not want to mount with sync, so I watch my writeback cache with this to ensure completion:
alias watchsync='watch -n 1 -d grep -e Dirty: -e Writeback: /proc/meminfo'
@PerseverantX please also include the results of cat /etc/fstab and sudo smartctl --all /dev/sdX, where sdX is the disk in question.