Maybe the instrument was given a wrong Expression map (XM)…? dynamics for instruments like piano and harp are controlled by key velocity (XM “Default”), but sustaining instruments are controlled by a continous controller. (CC1 or 11 mostly). You could try to change the XM either to “Default” or “Modulation Wheel Dynamics” (depending on what D. has already chosen for you)
Edit: there are a few Halion instruments which, as far as I understand, don’t respond to either of those, but require Note Expression information, which isn’t yet available in D.