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

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!