Thats to be expected with rear torsion beam setup (mainly to cut cost and have better rear leg room). Go for multilink setup if you want stability + comfort, but may have slightly less rear leg room.

Higher ground clearance doesnt mean stiffer ride. But since HR-V is a crossover SUV, it will more likely to have a stiffer setup than altis. From my experience, altis is very soft and floaty especially on highway speed. Good for low speed city driving but definitely not for high speed stability. But having the same torsion beam setup, going over bumpy road will be more "unsettled" compared to multilink setup. Just drive slow and it will be fine though

Bumpy or not its all depend on the setup (be it multilink or torsion beam). But torsion beam will definitely fell more unsettled going over bumpy road or pot holes compared to multilink as it allow independent movements of the wheels on either side, while torsion beam ties the wheels together to a certain degree.