GoForward goes back when it’s used with <PageControl>
This is just a case of unfortunate naming (or possible the names being chosen while only considering HierarchicalNavigation). When dealing with LinearNavigation (such as PageControl) you should instead consider “Forward” and “Back” simply as “Direction A” and “Direction B”.
Sorry for the confusion here. We’re evaluating several parts of the current navigation system and it’s quite likely that there’ll be updates and clarifications for things like these. In the meantime you can hopefully get by with imagining new names for these actions. 
(Or you could of course apply your own names by subclassing them. E.g. <GoForward ux:Class="GoLeft"/> )