Please try without any custom profile first, because if that profile was made before we added support for the 737-800 variant, the profile is likely wrong.
I see this sometimes, not every time. Which is a bit puzzling. No other profile except the built-in PMDG profile.
I've wondered if I maybe interrupted the 'flow' of the right-side FMC press macros, even though I keep to the left-hand FMC when GSX is in use.
Not 100% sure but I believe i've only seen it happen with stairs boarding mode.