As an update ...
I contacted Apple Support via Chat
They got me to add a new user (admin access) to test
When logged into my new tea admin user account ... the random reboot / shutdown thing doesn't happen
Also ill note that I was able to successfully rename the AppleThunderboltNHI.kext item but this did not fix the reboot issues.
Getting back onto support now that I have reinstalled Big Sur and issue still occurring in my main login
I am confused why it is only an issue with my main account ... but a new user doesn't have it ... interesting!! ... will keep you updated
MickH
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Having same issue and have tried also the remove the directory.
Would appreciate a fix too as totally stuck after hours of trying to solve!