Zoom to the first couple of seconds, then use Actions - Fix data, and apply a function for Elevation where you set the visible part to a fixed altitude. I used 100 in this example
This will correct the first second(s) of incomplete data from your device. My Coros watch has this same behaviour and logs 0 elevation until the GPS signal is locked and I’m moving.
Another option is to crop the activity so that the first second(s) are no longer part of it. Zoom to what you want to keep and the use Actions - Crop.