If it's coming disconnected then it hasn't been fitted correctly or the plastic so called 'quick fit coupler' is broken, like my old one in the photo i posted.Because this is the second time it falls off. The design of this thing is horrible, and doomed to fail. I will very happy to blind it off and remove the filler.
I bet there are dozens, if not hundreds of cars out there with incorrectly refitted oil filler pipes that are doomed and destined to leak.