Voltage drops 200mV on Tlora1 battery powered as current demand increases

I am testing the current demands of a Tlora1 node battery powered measuring voltage and current absorption with INA219 sensor in real time and noticed that when the device requires more current (from 68 mA waiting for messages to 180 mA when transmitting) the voltage drops by 150 - 200mV (i.e.: from 4V to 3.8) then the voltage return as it was when the current was 68mA. I wonder if this behavior is reasonable or not because I thought a 3500mAh Li-ion battery is capable to supply a relatively great amount of current with no voltage drop at all.
Here is a sample graphic showing the issue