Newer gliders with powerful air brakes will lose airspeed rapidly in the flare. Flaring too high or from an inadequate airspeed, especially with powerful air brakes full on, can end in a hard landing.

Perhaps the tailplane in the taller gear variants of the DG-1000/1 produces more lift as it rotates around the main gear to cushion the tailwheel contact.

A less tall gear does increase the possibility of a tailwheel first landing.

Another consideration is the amount of tail ballast carried.

Have you noticed if there's a difference with heavier or lighter pilots in the front seat?

It might be useful to have two experienced pilots, one to video and take notes, to do landings at various airspeeds and CGs to determine optimal landing parameters - and determine what to avoid.