Android : Button with link to #page or index.html#page doesn't work on Windows Phone

I have two buttons in my app with the following code:

<a href="index.html#index" data-role="button" data-icon="arrow-l" data-iconpos="notext" class="ui-btn-left" data-transition="flip" rel="external"></a>

and...

<a href="index.html#lesbrieven" data-role="button" data-icon="arrow-l" data-iconpos="notext" class="ui-btn-left" data-transition="flip" rel="external"></a>

On iOS and Android this works, but on Windows Phone the button doesn't do anything.

What could be wrong?