How to prevent Windows 10 from immediately waking after shutdown or sleep?

After newly setting up Windows 10 on a new SSD, it always immediately wakes up again if I shut it down or send it to sleep. The only way to keep it off (aside from cutting the power supply) is to forcefully shut it down by holding the power button for a few seconds.
Not that long ago I also purchased new RAM.

I am not sure, if this is a hardware or a software issue, and would be appreciative for any clues.

For hardware, I run different native Win-10 checks and got no errors:
SSD:

  • Drive Status in Security and Maintenance is OK
  • chkdsk offline scan with restart did not find any errors
    RAM:
  • Windows Memory Diagnostic Tool did not find any errors

For Software I ran down the complete list to make sure that Windows does not wake on its own as described in Conclusively stop wake timers from waking Windows 10 desktop

  • “powercfg -devicequery wake_armed” returns NONE
  • I successfully followed all the other steps to prevent Windows from initiating the wake-up
  • “powercfg lastwake” returns “Wake History Count – 0”
  • Event Viewer after sleep, regardless if I manually pressed sleep or if the system was idle long enough:
    • Power-Troubleshooter Events state that between sleep and wake lie around 15 seconds and have always “Wake Source: Unknown”
    • Kernel-Power Event-Id 42 entering sleep is always followed by Event-Id 107 resumed from sleep which happens most of the time 1 second later (sometimes it is up to 20 seconds later)

Windows-10 Pro is the only installed OS, and besides the SSD I have a HDD containing only data.

Has anyone an idea in what direction to go from here?