One thing you have to remember though, if you are burping a lot and you ruled out other GERD or physical damage like the LES then its something below the esophagus.
It is either in the intestines small or large. It may be probiotics don't always rely on the test, research and see how good are the tests. Keep testing, I mean if you have symptoms and are not feeling better than your body is telling you something is wrong.
Burping is my main symptom and I am currently trying to get rid of h pylori. Keep testing and monitor you supplement levels like vitamins levels.