Unchecking the "Power Off UPS" value in UPS Service still results in a UPS getting shutdown.

modifying this value updates /etc/nut/upsmon.conf appropriately:

POWERDOWNFLAG /etc/nokillpower

vs

POWERDOWNFLAG /etc/killpower.

But the shutdown command located at /usr/lib/systemd/system-shutdown/nutshutdown does not check this value and issues a /sbin/upsdrvctl shutdown command regardless.

This shutdown script should check for the existence of the /etc/killpower flag to determine if it should run that command, one possible fix would be:
if [ -f " /etc/killpower " ] ; then /sbin/upsdrvctl shutdown ; fi