Well...I finally found out why Luca dips his head when being ridden. When going that one way, there is that tiny uphill part of the circle so I tend to lean back too far and land not-so-softly in the saddle (rising trot). So I've now practised my rising trot to make it gentler and more supple and IT WORKED, no head dipping anymore. Also, when trotting in hand, he doesn't do it either. Weird....maybe it's because I corrected him every time he dipped his head like that. Anyway, I just wanted to tell you guys how glad I am that this issue was resolved.