Wrong battery type set is a good bet as Rob says Mike.
The other potential problem is overheating.
Extract from your ESC manual-
"● Protection for overheat: the output power will lessen to half when the temperature reaches 100°C during the operation of ESC.The output power will automatic recover when the temperature is under 100°C."
With your 19.2V (Nominal!) You are operating at or near the limits of both the motor and the ESC.
Bear in mind that when fully charged your 19.2V battery might well be actually at 22.4V.
(Circa 1.4V or more per cell instead of the nominal 1.2V)
Your meter readings indicate a total of 21V, presumably after a test run?
Thus causing higher current to be drawn and consequent heating.
Do a 'finger tip test' on the next run 😉
Just a thunk 🤕
Cheers, Doug 😎
T33; this ESC does appear to have a battery type selection function.
But strangely NOT via the programming card?
Extract from ESC manual-
"III.The function of electric speed controller for brushless motor
1. Reset to factory default
2. Battery type set (Lipo or NiMh/NiCd)
3. Brake set (yes/no)
4. Threshold set of protection for low voltage (set a protective value for low voltage)"