Unfortunately, several of the public "testing" sites have been generating
false-positives.
It would be good of you to list those here if you know which ones are
generating false positives, so folks can avoid using them.
Under the right (or wrong) conditions every public DNS testing site
has generated a false positive for some complex DNS configurations which
the test creators hadn't expected. The maintainers of the public testing
sites have been "patching" their tests to better handle some of problems.
So the testing results may change from day to day.
You need to understand how both the test works and what the results mean;
not only whether it says "Pass"/"Fail".