What they're saying is a load of shine. There's no way using any gears can cause the retainer to come undone. As I already said, it has a locking spline. It can't come undone by itself, and if properly tightened is pretty hard to get off with the correct tools. For that reason, I never tighten mine more than enough to nip it up - maybe 10 Nm instead of the specified 40 Nm. You can feel when the spline starts to lock. I've never had one come loose.So chain checker came in. Still all good as it doesn't through 0.5m so can only assume its the cassette. Here's the following response from the bike shop. Looks like i might try out a new cassette. Ebikes tend to go faster than normal bikes so using the lower gears will be likely, Rarely a need to swap to different gears as the motor will take care of any steep hills and such, seems like a catch 22.
Having reviewed the photo of your cassette i can clearly see you are predominantly using the bottom 3 sprockets which as i mentioned in the previous email this will greatly increase the wear on the cassette and chain if the full range of gears are not being used. Regardless of the age of the bike we would expect the customer to pay for worn parts. The strain of the motor will have caused the cassette to loosen over time as the pressure from the bottom 3 sprockets being used exclusively could cause the lock ring to loosen slightly.