There's nothing wrong with the holding assembly. In order for it to release, it needs a signal telling it that the blade and reached the feather position. It's not uncommon in aircraft using the hydromatic propeller to have the button fail to pop out, and accordingly the feather pump continues to run and the prop can drive back our of feather.
This is not complex. Neither is it unreliable.
One puts one's fingers behind the button and is ready to pop it out in case it doesn't pop out on it's own. This is why a cutout is made around the button guard on aircraft equipped with them...for that very reason.
Guarding the feather button isn't rocket science. Just good procedure.