Eco-start almost never operational
In reality, the alternator is barely capable of sustaining the systems if you are using everything at a low idle rpm, so to shut the engine down there has to be a good surplus of energy built up in the battery. If you were running at a nice crusing speed on the highway and it had built up a comfortable cushion and you came to an exit and stopped, with the a/c on low and not every electrical consumer running, it will probably shut off for 20 seconds. Turn your a/c off and it will shut down all the time.
One more thing to try. After the engine is at running temp (190-200 on the temp indicator on the lower right side of the instrumentation), when you bring the car to a stop, hard press or double press the brake pedal so the car is in brake hold (Hold is displayed on the instrument cluster). you should be able to take your foot off the brake pedal and the car is still stationary. Then if the eco-stop hasn't kicked in, bump the gear selector up so the transmission is in N (neutral). This combination triggers eco-stop most of the time on my 2015 S550W4, even with the A/C on.
I find that on my car it feels like the transmission sometimes isn't fully disengaged at a stop and I can feel a slight vibration when it's in drive (D2) with the brake held. Bumping the gear selector to N fully disengages the drag from the transmission so the engine runs completely smoothly and the eco start seems more likely to trigger.









