You don't have any advantage in Solasta when you're higher. You both know the rules better than I but if I remember well it's not written that you should have any bonuses if you're higher.
That said, as Mrfuji said it was REALLY an issue in BG3. The difference between death and victory. Now the verticality of the map is interesting and the bonuses/maluses are balanced.
This change was a great move and in my opinion it's probably one of the only combat mechanics in BG3 that should inspire Tactical Adventure.
