Propshaft moved forward under full throttle

[Pauls]

As Bilknny said, we need details. Specifically, your shaft has either a straight diameter where it mates with the coupling or it can have a taper which mates with a matching taper in the coupling.

If you have the taper setup the coupling is held on the shaft with a large nut on the shaft end. That nut would have to be very loose for the shaft to move.

If you have the straight shaft/coupling setup (and you probably do) then the shaft is prevented from moving forward/aft in the coupling by bolts in the coupling which tighten against the shaft. If these bolts are loose the shaft is free to move. In reverse it can pull completely out of the coupling. Ideally these bolts should bear on flats machined into the shaft. And they need to be really tight. They should be loctited or safety wired.