Throughout my gaming session last night from 7pm to 12am, my mouse (Razer Cobra Pro) has been stagnant at 12% and while it was charging, I would constantly check on the battery life to see if there was any increase but it decreased from 12% to 7%.
Please take note that both the wireless charging mat (Razer Hyperflux v2) and the mouse (Razer Cobra Pro) are new.