There is a phenomenon known as voltage sag and it happens to all batteries in certain situations. If there his a high demand for power, for example: If you are going up a hill or are in pedal assist 5 using 95% power or throttle, and or riding into the wind, this will consume a lot of power. The voltage read out of the battery while this is happening is going to read lower than it actually is and will read low for some time after. If you stop at a red light, you might see it come back up a little in just a minute.
The amount of voltage sag, or how much lower it will read depends on how much power was being drawn from the battery and sent to the motor. Essentially, we cannot read the voltage accurately to determine how many amp hours are remaining while there is a large demand for power from the battery. Once this high power draw stops, the voltage will adjust slightly and come back up. For example if you stop riding for a while or even if using 800W of power, and then switch to pedal assist 1 and am only using 250w of power, the battery may start to read more accurately. Once you come to a stop, and let the bike sit, in 1-10 minutes you may see the voltage rise to its current accurate voltage.
When the voltage sags like this, if it sags too low (below the required voltage for the controller and motor to operate), the controller will stop sending power to the motor until the voltage returns to normal. On a larger battery, you are less likely to see this, on a medium or small batter, as you run the battery percentage down, you are more likely to see this. So if this happens while you are riding, and your battery is low, it is probably best to lower your PAS to level 1 and continue your ride in that fashion. If the bike powers down because the battery is too low even in PAS 1, it is best to not turn the bike on. Return home and charge it as soon as possible.
Here is a 3rd party take on it from the EBR Forums: https://electricbike.com/forum/forum/knowledge-base/batteries-aa/48140-voltage-sag-explained
If you understand voltage sag, but are still concerned about your batteries performance, it may be best to test your battery range, use the trip meter on the bike. There are many variables that can contribute to how efficient your ebike runs, including the temperature you are riding in. Cold temperatures near freezing can lower range and you may experience voltage sag sooner than you normally would. Other things to consider: Charge the bike to 100% (the charger’s LED should turn from red to green), make sure your wheels are both spinning forward freely, no friction from brake or anything else. Tires inflated to max PSI (listed on the side of the tire). Soft tires will reduce range and increase rolling friction. Ride with the bike in pedal assist 1 and pedal hard or moderately hard and test out your range with that on relatively flat smooth ground. Did you hit or exceed the expected range?
The biggest challenges to range are typically: underinflated tires, poorly adjusted brakes, riding into the wind, and riding up hills. Each of these can really impact your range, as they require more power! This is a good way to confirm if what you experienced, was indeed just voltage sag, and that your battery is giving you the expected range for your model.