I have checked the "Log out after this many minutes of inactivity" box and set it to 30 min and it is constantly getting reset back to default of unchecked (which is a poor "default setting", IMO).
I searched this discussions and one answer was it could be due to cookies/cache/browser settings being reset. I have not made any changes. Do these extension settings sync between installs? I have LastPass browser extension on multiple computers (Mac and multiple PCs). Is the change being overwritten by another computer's settings being uploaded? That would be pretty poor if an older config could overwrite a newer config.