Hello. Immediately after this happens, open Event Viewer > Windows Logs > System. Around the shutdown time, do you see Event ID 41 (Kernel-Power), 6008, 1001, or 1074? If you see 1074, what does it show for "Shutdown Type" and the process that initiated it?
Also, try this:
Open Command Prompt as administrator and run:
shutdown /s /f /t 0
Does the laptop remain powered off when shut down this way, or does it still occasionally power itself back on?