The pedals go to sleep after 4 minutes. I would assume that is it, but weird that you didn't have the issue with the 500? Once you spin / move them it reconnects. So your garmin isn't getting power for the first 30 seconds or so but should after that .

I have the 520 and P1s, calibrate in the morning, turn off auto sleep on the 520, go swim, get on the bike and pedal, power shows up