The two threads linked above clearly explain what problem is, and why it's a P3D bug. Which should be pretty obvious consider, as you said yourself, it was fixed by acting on a P3D file, unrelated to GSX.
Basically, the Html menu in P3D is still not reliable, it was disabled in P3D 5.2, because it wasn't stable enough. Apparently, is still not entirely fixed, that's why reverting to Scaleform fixes the problem.