Is there any other electrical equipment that does not work when this happens? There is a load shed strategy when the 12 V battery falls below a certain level (IIRC somewhere around a 40% SOC). I have never seen a list of equipment that is shut down when this happen but know the Infotainment Systems are shut down.
I would assume that the charging algorithm for the 12 V battery is temperature compensated (like the voltage regulators in alternator charging systems). Lead acid battery life can be increased by lowering the charging voltage when the 12 V battery is subject to high ambient temps. We also know that the 12 V battery data is monitored via a sensor in the negative cable. Perhaps the battery is failing and the SOC is below / near the 40% threshold on startup and charging via the DC/DC converter is limited because of high cabin temperature. Then, perhaps, the load shed program is initiated.
One other comment. There is a sun load sensor in the center on top of the dash. It can be pulled out as there are spring clips on the sides. I know this sensor affects the operation of the climate control system but maybe its data is used by other modules. Try covering it or perhaps disconnecting it and see what happens.
I'm going to say this again (as I've said in many threads) that there may be DTCs present in the modules. The ForScan App could be very helpful in checking modules for DTCs and monitoring data from the modules during this condition. Is a module "dead", not responding, what is its status, is there a broadcast load shed message to the modules, or something else happening?