I have the same issue long time ago. XF's Team bring the 'adblock'/'script block' plugin on browser as the cause. But I fear that is not the proper approach to this cookie case.
Specially because the majority of the visitors prefer not to start a new profile in (any) website, every single browser have an adblocker feature enabled by default, and they want just to read the page without dealing with login/register.
Well, my solution, against the law, is to disable the cookie notice until a change that just permit the visitor to dismiss the notice without any redirect to the login page.
Or I'm completely wrong and there is a better approch to this.