This is anecdotal but reproducible. I rode my wife's gravel e-bike about 10 miles, mostly uphill (about 2000 ft total climbing), today. I made a point of taking it easy. My average speed was under 10 mph. I was in "tour" mode for most of it, but on two steep hills, put it into "sport." I didn't really break a sweat. But what I did get was a leg cramp, which is still persisting, 4 hours later.
I've had this happen before on this bike. The fit is ok but not perfect. I can ride her other (non-e) bikes with no issue.
The only thing I can think of is that the e-bike encourages/demands the rider to constantly spin. If you stop pedaling, it quickly comes to a halt. Restarting is about as much fun as doing so on a touring bike. For those reasons, I tend to spin continuously, which reproducibly gives me a cramp. Oddly, I only get it in the left leg.