2005 Prius with P0AA6, Subcodes 526 and 613

Our 2005 Prius recently refused to start and gave a P0AA6 with Subcodes 526 and 613.

Normally, this indicates that the transmission needs replacing according to Toyota's Service Bulletin.

But here's the weird stuff:
The thing will set this code when the transmission is cold (unusual according to LG).
It has also never done this before, making it weird that we'd have a problem fairly consistently and, the code REAPPEARS when cleared with the car NOT attempting to be "Ready".
The 'LG' test would suggest that the issue lies in the HV battery in the rear of the car, not the transmission.
After all, how would the car detect a HV fault in the inverter, or MG1 or MG2, without any high voltage present?

Is this a computer error? The car is getting up there in years, and we were not the first owners. :/ So unfortunately anything is possible.

If I clear the codes and attempt to READY the car, it works, and you can drive it. Warning lights sometimes reappear, sometimes they don't. Almost invariably, the car will not enter ready mode again (even if no errors were displayed during the previous run) after shutting it off, until codes are cleared again.

I feel weird proceeding either direction, as the Lucious Garage test is suggesting the fault is with the battery, but the Subcodes are suggesting transmission failure. In my previous experience, those tests always agreed with one another.