accordeon stacato and legato mixedUp

I want to have the melody played (G clef) legato and the accompaniment (F clef) played stacato, but
at the moment the playback of the melody follows the stacato of the accompaniment.
with either GM or samplebased stock HalionSE sounds

Yes, if you have two voices on the same staff, one marked staccato and the other not, unfortunately at the moment both will be played back staccato. This will be improved in Dorico 1.1.