My Edge 840 (non solar) will wake from sleep/standby (short press power button) after a few minutes, which is a problem as I have auto sleep disabled (meaning the battery will just run down to zero). To stop this happening I have to long press the power button and fully power it off. The unit is not plugged in to a charger. Firmware is latest.
My Edge 530 didn't have any problem with this, and would stay in standby for days without any problem.