This misunderstanding is one of the reasons why people think that hypnosis is dangerous. It’s hard to say how this misunderstanding was conceived, but I guess that some stage hypnotist couldn’t wake a person up in front of an audience. One of the reasons that stage hypnotists can sometimes have trouble eliminating the trance is because hypnosis is such a pleasant state of mind. It would probably only happen to the inexperienced stage hypnotists, as there are many ways to terminate a trance.
But even if an inexperienced hypnotist cannot wake the subject up, it doesn’t mean that they will stay in hypnosis. We enter the hypnotic state several times during the day, and we get out of it as well. So if a hypnotic subject cannot wake up, the stage hypnotist can use some techniques OR simply let the person stay in a trance. The issue will wake up eventually.