I can reproduce this every now and again, when triggering AMRAAM on weapon select, when on the ground. The HUD goes into navigation mode, the instant the aircraft starts moving. If you weapon select in this small zone of initial inertia, the HUD makes a small change but you cannot select a weapon. By looks of it the A/A mode is disengaged, selecting a weapon will not engage A/A mode. This effect gets canceled out once the velocity goes above 20 knots as far as I can tell on SP (not sure why, you're still on the ground), however, on occasion, it sticks. I was noticing it on MP, single player interestingly, regardless of what I did, I could not reproduce the effect. Flicking a/a mode on and off does resolve it when it happens though. However at the very minimum, being able to select A/A on the ground was a recent bug fix, if the initial movement of the aircraft turns it off, seems it would add to pilot workload if in real life and counterintuitive.
I do have a track but its from a rather extended gameplay session on MP and I imagine a bit of a nightmare to navigate. I will try next time I am playing MP.