Methinks some code that you think is running is not. A private function in a form module should be available to all instances of that form. Step through your code and make sure it's doing what you think it is, or copy db, compact, zip and post here. Remove all but what is needed to demonstrate your issue if that helps.
EDIT - best to copy & paste code to eliminate some questions (please use code tags (#) on posting toolbar). Can't tell if you've declared your form variable at the module level or not.
Also, I've tested now and experience the same thing. However, the built in navigation controls work independently so foregoing custom navigation is probably how I'd fix this.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.