DG is quite correct with his first statement.
The ESC and motor don't care where the electrons come from, as long as they get some.
True a fully charged 2S LiPo
can measure around 8.4V OFF LOAD.
But as soon as you put it on load, especially high current as the ESC is rated for the cell voltages will quickly drop to the nominal 3.7V, =7.4V for a2S. That is perfectly normal.
The snag here is that your ESC has a Low Power Cutoff built in.
Since it is designed for NiCads and NiMh of nominal 1.2V per cell it will assume that your fully charged 2S Lipo is a 7 cell NiMh, i.e. 8.4V. It don't know nuffink about LiPos🤔
The lowest safe discharge voltage of an NiMh cell is 1.1V, x7=7.7V
Therefore I would expect that the ESC would cut off (or slow down perhaps?, not clear from what I can find in the WW-swamp) as soon as your LiPo drops below 7.7V when in fact the battery is still almost fully charged.
I don't think that situation will bring you much 'Deep joy!'☹️
If I were you I would use a decent Hi-Power NiMh of suitable volts and capacity for the motor(s) you are driving.
Cheers, Doug 😎