I believe it’s an issue of being an open (outlined like a half note) diamond vs closed (completely filled in, like a quarter note). It’s a widely used practice that no matter the note length of the harmonic diamond head, it should be open like a halfnote. So far I’ve not seen a way to force that option. Yes, it’s fine for half note/whole note harmonics, but for shorter notes we have to live with the filled in diamond head.
There may be a workaround using other voices for the diamond, so it’s length can be a half note - but for now I’m content to just leave diamond heads as they currently are until a true harmonic notation is added. It still gets the point across.