I have done some extensive testing on this now and it is a problem. recently I started getting issues where the wait for response was being bypassed. The user was getting the email, but the response had already been decided in the flow. The response is an inconsistent random selection of the buttons.
I found that when I set "Show HTML confirmation dialog" to yes in the advance section this problem went away. This was not a mandatory requirement and until recently my flows using the action were not a problem