...my only guess is that the audio transfer from your DAW comes in at a lower level, so you might want to check your audio routing. Also, make sure Melodyne is in the very first insert slot of the respective mixer channel. Any preceding plug-ins might blur the clarity of your audio and thus confuse Melodyne's detection algorithm.
If this doesn't help, I'm pretty clueless for now, so let's wait what the support guys say.
Nice guess Claudio, but this is not the case - the mixers are all the simplest possible, containing the Editor Plugin only, everything, including Return from Standalone, at Unity gain. I even checked with files of pure tone to be sure levels equal. The behaviour of the 3 VST hosts is exactly the same.
Support have been in touch but don't seem to have a conclusion yet......