Just to be of potential help: did anything change in regards to the settings of your WLAN router your Switch is always connected to lately? When I had to reset my device, it also reset the password and name of the device of course. For some odd reason however, the Switch will still try to connect to the old access point, which obviously doesn't exist anymore, repeatedly until it's able to successfully connect to it, so it's basically stuck in an infinite loop which drains the battery incredibly fast even in sleep mode. Deleting the old access point of the Switch fixed it.
Hope this can be of use for you, would be a shame to have the issue be tied to something we aren't able to work on ourselves.