Solution!! (for now at least)
I just got the iPad Mini 6 and have been experiencing this issue as well. I am currently using Touch ID, and even though the iPad states “Triple Click to End Guided Access”, my iPad will freeze up if I do that.
My solution that has worked every time since discovering it, is to double-click the Top Button(TouchID/Lock/Home Button), follow the second click with immediately resting(not clicking) your TouchID enrolled finger. iPad Mini then seems to unlock without an issue.
Update: I have noticed if you wait too long after the second-click of the TouchID/Lock Button, that this workaround is less consistent with being successful.
I’m sure in the next few weeks, we will see a supplemental iOS update that will fix this bug so guided access works normally again.
I made this short clip to help demonstrate this workaround. (YouTube link below)
https://youtu.be/WbMNPNRu0no