Sounds like a loose wire. I'm guessing the wire from the display to the motor. You can probably fix it yourself but it's a little bit of a pain. You have get in there find the connection and I would reconnect the wire make sure it works and use some lock tight on any screw threads.
I love my Decoy, parts break and ebikes are expensive. Great to hear about the service. I hope that one day Shimano will make their systems serviceable instead of throw away when they fail. Bosch is going this route. I have never ridden a Bosch but I do love the Shimano.
I threw in the 180mm spacer for the 38. Right now it's almost perfect with the 0.5 - 1.0⁰ change. Plus the extra travel on the front and bit more pedal clearance. It's tits.