When I switch my laptop to energy saving mode, it sometimes reboots when I haven't been there for a while. I haven't seen the restart myself, but I'm always on my lock screen. I think it came from a new Windows update or I was hacked.
I didn't choose sleep mode either, so I don't know what the problem is.
Most likely: It is due to the Windows update settings. If you have not properly formulated the energy-saving mode or have set it clearly, there will be restarts when updates are available (and these settings are also not configured). You can catch up on that here: https://www.bestetipps.de/computer/windows-tipps/windows-update-einstellungen/
I would set it this way:
Energy-saving mode: everything always on, maximum output, never shutdown, maximum output
Windows update: you decide what happens. Deactivate everything "automatic" if necessary?