my garage fridge or freezer usually sits between about minus four and minus nine right now. the lowest temperature it has recorded so far is minus thirteen, probably from last winter during a real cold spell.
here is a test if you own one of these -40 freezers. set it below zero, unplug it for a minute, then plug it back in. check the display an hour or two later and it will likely be above the temperature you chose. a short power loss makes the unit ignore your setting and switch to its built-in default of zero degrees f instead. holding the plus and minus buttons together makes it obey your chosen set point again. the company sent a replacement but it did the same thing, then refunded the purchase and let me keep both units. the customer service was good but the freezers are not reliable. these units seem to be sold under different labels by several sellers so the same problem probably exists on the look-alikes.