Take off the switch machine and move the points manually. This should take almost no effort, and the points should stay where you put them. I think Eric has the answer on this.
One other possibility. Is the turnout mounted on a slope? These machines have a large metal slug which moves with magnetic action. If there's enough of a slope, when the magnet is de-energized, gravity might pull the slug back down in the other direction.
It takes an iron man to play with a toy iron horse.
I had something similar and I swapped out the machine part, checked the wiring to the switch and it still bounced back. The cause was some debris got under the track and I guess it caused enough pressure to spring it back. Try cleaning around the switch with dust buster and see if that helps.
On my layout I have several (20+) of these machines and I know a lot of guys hate them but I have made the financial commitment and am trying to get my money's worth. I have run into an unusual issue and need some ideas. I run the wires as per instructions but I do run the central common wire in series to several turnouts and they all work well except this one. When I hit the control switch the points go to the opposite rail but when I release the button the points return to the original rail. I thought I may have a defective machine and it has been changed out, still does the same. Maybe a bad control switch, replaced and same thing. Rechecked the wires. Can't think of what what I have missed. Anyone able to help???
Thanks, Alan