I noticed the speed of all cars (Mercedes included) dropped at the en of the main straight. The driver lifts off the throttle in a timed lap once he crosses the S/F line, but this happened during the race. I would think that the harvesting from the MGU-H would be enough to keep the batteries. The RedBulls were complaining about battery issues, and that makes me think that their battery was draining quicker than others. I wasn't aware of this speed drop until Baku, maybe because the straight is so long?
This is an example of a RedBull. It tops 320 kph but as it crosses the grid, speed begins to drop.