This isn't a bug with PPrune, but a known issue with "some" ads, whereby they are coded to redirect you to another site. ISTR There was a news.com.au article on it a couple of months ago and most times the Advertising brokers don't even know it is going on... Some scams then redirect you several times resulting in the "back" button having no effect. Gumtree is the same, I often get these ads there.
The solution to this, if you are running Android, is to long-press the Back button, and it should open a list of the last half-dozen or so pages you've viewed, and you can then reselect the Prune page you wanted to see in the first place.