Just be glad it went that way (heater off) and not the other way (heater stuck on).
Obviously they don't like being in the cooler temperatures, but they can tolerate it for a while without incident - like others have mentioned, they regularly hit temps like that while shipping.
On the other hand - if your heater gets stuck on, it doesn't take long at all to kill everything in your tank. My very first tank crash happened exactly that way - thermostat on my heater broke while I was on vacation, got stuck on and held on for hours. By the time I noticed, the temperature in the tank was around 99 degrees and everything except for a flametail blenny was dead.
That's why I run with three layers of redundancy on my heater control: The thermostat for the heater, a heater controller with its own temperature probe, and then my Apex.
Does your heater get exposed to air when you do your maintenance? If not, you don't actually need to turn the heaters off while doing a water change.