I am no expert in looking the logs, but here is what I noticed.
You should look at CURR.Volts.
Correct me if I am wrong, as I assume you have an 8s battery, the starting voltage was 33,29V (4,15 per cell).
In the moment you switched to RTL it was down to 25,6 (3,2V per cell) and on the peak of the RTL climb 24,66 (3,0V per cell) and then it dropped quickly 17,6 (2,2V per cell) while you motors tried to keep it in the air.
Finally, without the load, the battery was at 20,5 (2,56V per cell) which is way too low.
I would check the battery after this as the voltage shouldn't drop under 3,0V.