Does anyone know if this is expected behaviour for e-bikes of this type or if there is something wrong with mine?
The best place for information would be the manufacturer.
As a general rule, no this would not be "expected behavior". But, it can certainly depend on the exact implementation of the motor controller for each bike.
I have ridden a wide variety of e-bikes, and have ridden bikes with two different types of controllers with the battery at a low, nearly-discharged state of charge. So I can say how those motor controllers work.
One used a Bafang motor and controller. As it reached the discharged state, I noticed no decline in power, until the motor just started "cutting out" intermittently. That is, power was cut completely, until the battery recovered enough to provide a little more power. As you can imagine, this was really annoying.
The other used a Bosch Performance Line CX motor and controller. On that bike, the battery did not actually get to a state of discharge where it wouldn't drive the bike at all, and I did not experience any decline in power, in spite of the fact that the indicated range was 0 (i.e. battery level was "zero bars", and state-of-charge low enough that the display wasn't willing to provide any range estimate).
Based on the official reply from a Bosch employee in this online forum, it appears that had I managed to discharge the battery even more, it eventually would have just "cut out", the same way the Bafang implementation does.
So, in both cases, the available power provided as long as the battery had some charge available, was the usual amount.
But that's not to say that a manufacturer couldn't include a feature that reduced power as the state-of-charge declined for the battery. And frankly, given the stated specs on your bike — 70-100 km on just 352 Wh of energy, which seems highly optimistic to me, except at the lowest assist levels — it would not surprise me at all if they achieve this by significantly reducing the amount of assist (and thus the amount of battery drain) as you approach a lower state-of-charge.
If you're concerned about it, you really should just contact the dealer and/or manufacturer and ask them what the expected behavior is. If it's not working as designed, you're going to have to contact them anyway, so you might as well start there.
(As an aside: in the most basic implementation of an e-bike, it's even possible that the power provided is limited by the battery voltage, which declines as the battery discharges. But all the motor controllers I've seen are "constant power" controllers, i.e. they draw a certain amount of current from the battery to achieve the desired output, and on a bike such as yours I can't imagine they would use something different from that. So if you're getting less power, I'm confident it's the motor controller that's causing that, not just a reduction in battery voltage.)