Polearm master feat bonus action does not gain +2 damage benefit from dueling fighting style.

Title says it all but I will write it out anyway.

In dnd beyond, if you pick the fighting style dueling by any means available, the overview will only show the added +2 for your main attack, and not the bonus action that is available if you use the PAM feat. 

This has been reported many times. It's a limitation of the software that the Feat cannot know what weapon you're using and so doesn't modify the attack it puts on the sheet in any way. You would have to manually customise it by clicking on it and customising it from the pop-out window - or create a custom action for it.