Of course there is one more variable here that 'could' have a bearing on this intermittent problem. We're not dealing with FS9, FSX or P3D are we? Nor are we working with FSUIPC. Xplane and XSUIPC have to be factored in and I don't have either, so attempting to recreate the problem would be somewhat difficult if not futile.
Does XSUIPC have any logging facilities you could use and put in place to help pinpoint the issue? We can see from our logging that the relevant hotkey worked and as far as PF3 was concerned it executed correctly... the rest was then in the lap of the gods or XSUIPC to be more specific.
Just another thought
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