The first time you cold boot an Electron in SEMI_AUTOMATIC or MANUAL mode you need to turn the modem on before going to sleep. This is counter-intuitive, but putting the modem to sleep is done by sending AT commands at it, so if you don't turn it on, it stays half-awake because it doesn't get the command to sleep.
Note that you only need to do this on cold boot. I use a retained variable to know this. If the modem was previously asleep it will stay asleep.
This post is for SLEEP_MODE_DEEP but it also applies to stop mode sleep with cellular off.