I'm guessing but... Isn't it possible that the power adapter has 2 different outputs (5/15), because it's so intelligent that it choose what voltage instead you are recharging only the Switch or the Switch and 2 joy-cons?
Maybe it's absurd but think about it... Switch can be recharged with any type of usb C, not necessarly his power adapter. But what happened if I decide to recharge Switch and the 2 joy-cons with only 5 volt? Probably it works but first it recharge the Switch and then 1 joy-con at time.
What do you think?