Hello ,
I believe, Here Headless browser is HTMLUnit browser where we are passing the different version(like Firefox Version or Chrome Version)
Whta is Headless browser?
Headless Browser uses Rhino engine so if you will test some javascript applications then chances are high you will get diff result .Generally, Headless browsers are simulation programs , they are not your real browsers.JavaScript are implemented differently by different browsers. Although JavaScript is a standard but each browser has its own little differences in the way that they have implemented JavaScript
Problem:
It is not problem with xpath, it is just you have enable the java script before proceeding your test. So, to resolve this you can configure your headless browser in project setting. below is screen shot.
Please try this and let me know if it works.
~Prabhat