While I haven't used Logic, it sounds like the virtual OTQ VST MIDI I/O setting for diapason is listening for a MIDI event that Logic is obligingly setting to A440.  I'd suggest checking the specific VST MIDI routing for your OTQ instance and see if there's a listener that's editing the diapason and disable that.

If that doesn't work, "File > Project Settings > Tuning > Software Instrument Pitch" should allow you to set the Logic parameter that's passing A440 (though if you can get the other to work, so much the better).

Thanks for responding. I have looked and can't find anywhere either in Organteq or Logic's settings to disable. Tuning in Logic is a clumsier way than within OTQ. I normally use 440 so it's more an irritation that it is forcing a change I don't want. So if I can't find a solution it's not a major problem - just means I have to use Reaper or in standalone mode when I want to play around with diapason other than 440.

If that doesn't work, "File > Project Settings > Tuning > Software Instrument Pitch" should allow you to set the Logic parameter that's passing A440 (though if you can get the other to work, so much the better).

Just wondering if anyone has experienced this and solved the issue. I can just use standalone or Reaper when I want to play around with tunings other than 440Hz but that is limiting as I do prefer to work within Logic.