(Tested with FW 3.50 and 3.55)
Last weekend I did several laps at the Nurburgring:
https://connect.garmin.com/modern/activity/3888910650
The first four continous laps were fine, but after a break where the device went into 'sleep mode' the next two laps the elevation data (as well as the gradient durring the ride) was totally off and kinda 'muffled'. Gradients of 10% were now measured as 2% and so on....
Today I repeated this at home and the behaviour was reproducable: no correct elevation or gradient after the activity was stopped and then resumed. After I started a new activity all elevation data was fine again.