I agree with h-elsner about the battery in both telemetry files. You may have a problem with the charger also as there is only 12.4V at start of files and a fully charged battery should be 12.6V. I would suspect that there is at least one cell that has a high IR reading.
The graph above is of about the last 25 seconds of flight at a height of just over 72 meters.
1. @18:28:10 you get initial Low Voltage Alarm under 2 seconds with altitude 72.1 meters
2. @18:28:28.8 you get a consistent Low Voltage Alarm with altitude 72.4 meters
3. @18:28:32.7 you get initial 2nd Low Voltage Alarm still with altitude 72.4 meters
4. @18:28:43.3 you get a consistent 2nd Low Voltage Alarm with altitude 62 meters
5. @18:28:46.5 you reach a Critical Voltage level (red line in graph) voltage <10V with altitude 38 meters
This is the point where you hear the flutter as there is not enough power left to provide lift. Sorry for the loss of your Q Grumpy. You are not the first nor the last to lose your craft to a weak battery.
We would still like to see the Remote and Remote_GPS files that correspond to the telemetry.