This is due to the balloons using levitation to, well, levitate the mobs they're attached to.

ah, i see. The levitation causes other wacky things also. Like you could complete advancements that require having the levitation effect such as Great View up Here and How Did We Get Here.

Also known. Minecraft has levitation (or constant motion upwards) hardcoded so I just decided to use Levitation. I'm considering adding a hook for the balloons and not making it use levitation, but the hardcoding is deep-rooted in some if statements.