That is just Odd. Basically once you get the formula in the Section Expert under the details section it should create a page break whenever the formula evaluates as true. You have validated that the formula does do this and you are placing the formula in the correct location in the select expert.
The only thing I can think of left to test is to leave your formula field that you used for testing in the details section (post #5 called it 'Test Page Break') that gave you the instring # and try and change your new page after formula to use it instead... {@Test Page Break}>0
This should be no different than the other but can't hurt to try it.
Anyone else see what is missing here?