Not sure where this was introduced, whether systemd, grub2, or cent7 itself, but by simply adding data=writeback in to fstab for / will cause / to go ro with the message " Cannot change data mode on remount mount: / not mounted or bad option writeback"

This can be resolved by telling grub2 about the writeback option to the grub config rootflags=data=writeback

/etc/sysconfig/grub

GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
GRUB_DEFAULT=saved
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="crashkernel=auto rhgb rootflags=data=writeback"
GRUB_DISABLE_RECOVERY="true"

and recreate the grub2-config

grub2-mkconfig -o /boot/grub2/grub.cfg