You have to do a few things to get something like this to work. First, set your characters movement mode to flying while using the ladder, then add to the positive z value while in contact. Once you stop using the ladder reset the movement mode to walking/falling based on what is most appropriate for your setup.
This is the setup I use when I overlap my ladder actor to set movement.
EDIT: I just reread through your post and saw that you are already doing this. As far as I’m aware this is the best possible method to accomplish this task at present. Try setting it to falling instead of walking to get them to drop back down to the platform before moving.