When set Stab P or Rate P,I or D for Roll/Pitch tuning on channel 6 it’s value overrides autotune value for that parameter so it has no sense to set any of autotuned parameters to ch 6 while autotune is set to ch 7 or 8.
That sounds like it shouldn’t be a surprise that this is the case. I imagine it would need some weird logic to lock out the chan 6 if auto tune has been used, that could possibly lock you out from using channel 6 when actually required. Switch off chan 6 
I mean that there should be a warning when autotune set on ch 7 or 8 when one of autotuned parameters is binded to ch 6