This part of the provincial boundary was very likely traced by hand purely based on guesswork (see history version 1 from 2014). It would be great if you could improve the alignment to be more accurate. 

By the way, are you using any sources other than existing openstreetmap data when adding municipal boundaries? If yes please always mention them in your changesets.

In this case it seems that the actual boundary of Pigcawayan is somewhere in between the currently mapped provincial boundary and the municipal boundary you added. Have a look at the area around Panatan which is part of Sultan Kudarat municipality.

Boundaries of Barira will need to be revised too, there's currently a big overlap with Pigcawayan.

Thanks for your efforts!