Same exact issues here with the 750w Grand. APC and Cyberpower UPS units -- conventional and the PFC (pure sinewave) models. Spoke with a tech at Cyberpower and he said that it's an issue with Thermaltake and they've contacted them in hopes of a fix, but so far no communication from TT.
Came here to see if there was a fix, as some adjustments can be made to the voltages on the UPS backup but no answers to be found and no one picks up the phone there so I'm about ready to replace this unit with another brand power supply as I need a power backup solution.
Good luck to you all solving this!