Have a look at the battery voltage in your log from the copter.
You are most likely experiencing voltage sag under load.
By the time you measure the voltage on the battery it has had time to recover.
What’s the current draw of the copter? (again from the log)
What is the C rating of the battery?
What voltage do you have the battery failsafe set to?
I never use battery failsafe via Mavlink; the power modules I’ve used don’t read correctly under load. Instead, I use the FLVSS battery monitors that are available for the Taranis which plug in directly to the balance plug and provide a way to monitor cell voltage real-time. I also have one of the current sensors that use the smart port as well. I have an alarm that warns me when I’m down to 3.7v per cell. In flight the voltage reading is a little different than when at rest.
And a 10C battery might be borderline. They sag terribly under load. All the 6S batteries I’m using on my Hex (5 kilo rig) are 25C or better. Less sag.