I understand you cannot reproduce it, but it doesn't negate the fact that it is happening
I'm not saying I don't believe it's happening to YOU. But since it doesn't happen here, and nobody else reported it, it's clearly some kind of problem in your system, possibly caused by ANOTHER add-on ( not FS Labs / PMDG ) you have but I don't and probably not many others have as well, otherwise if it was common, we would have plenty of similar reports, not just yours.
Of course, I can only do a test based on what you report. You said GSX doesn't work with those two aircraft, so I tried with the one I have installed now ( I'll try with PMDG as well ), and it doesn't happen so, unless you can provide with more detail about other add-ons you have installed, I can only repeat and confirm that, GSX with FS Labs and nothing else installed surely works with no issue.
FS Labs has made a very complete integration with GSX: don't you think if GSX "really" had a problem with it, they would have told us, since I guess they must use it very extensively ?
The latest version that is available for download from the Lockheed site includes HF1, does it not? 5.1.12.26829
Yes, that's the latest version. You are now saying you are using the latest installer for P3DV5, that's why I had to ask, because you only said it was "a fresh install", without saying which version of P3D5 you used.
You must understand the ability to get supported, is directly proportional to the accuracy/completeness of your reports.