Hanna is infuriating. They apparently know that their reagents are defective, but don't bother advising consumers who bought them that there is a problem. I understand from posts on another forum that their alkalinity reagents are even more unreliable.
Putting all that aside, I find that the basic protocol for conducting PO4 and ULR PO4 tests to be ridiculous. The reagent "sachets" are too large/awkward to get ALL the reagent into the test tube on a consistent basis. Why not use a bottle of reagent and a "scoop" spoon as so many other test companies use? Then, there's the 2-3 minute deadline for dumping the reagent into the test tube, recapping it, shaking it until the reagent is completely dissolved, gently tilting the bottle to remove air bubbles, which IME is flat out impossible on the non-ULR test and needs careful and repeated training to make it work on the ULR test.
Bottom line, I would not be too confident that the results you're getting from your Hanna PO4 (or alkalinity) checkers are reliable.