...and I forgot to add.
In the past 2 months, Phase One has sent me three new P21+ backs to try to solve this issue. Every single one of them has malfunctioned in the *exact* same way. This can only mean one thing. It's a production issue from Phase One, and not an isolated case. So since their release, I can only assume that not a sigle P21+ back can do a one hour exposure at 57 degrees F, as advertised.