There is a function called "Soft Power" in the mixers, which can also be activated via UC Surface. The manual explains what this does:
"When powering down your mixer, you have the option to engage Soft Power. This will automatically engage Autostore to save all your mixer settings prior to powering down when 'Confirm Power Off' is selected.. You will also be given the option to log out of the current User Profile ." (StudioLive_Series_III_OwnersManual_V6_EN_10062019, section 12.5 Soft Power)
I can only recommend to use this function. Any "hard" powering down can possibly cause problems. Losing aux outputs is not a normal behavior of StudioLive mixers.
Try the Soft Power function and find out if that helps. If not, contact Support.