I have a memory of experiencing this as a child, but I can’t remember ever feeling it as an adult. I would say it’s more likely that the nervous system adapts to it.
I seem to remember the trampoline producing some odd stomach feeling, but I think that was more on a full stomach.
I think wolfpup got it:
A rollercoaster is attached to the track. When moving at a great speed, and going over a hill, it does change directions (from going up to going down) faster than gravity alone would pull it.
I’m no physicist, but I think inertia is more relevant than gravity here.
Another example is how I sometimes hear a weird slosh sound from my tummy when I move suddenly. Usually, my core will have everything sort of held in place, as said, loosely suspended but the odd lurch with the right stomach contents leaves enough headroom (?) to splash around, audibly.
That’s it! It’s the change from greater than normal “gravity” to none at all. Simply going from one to zero Gs doesn’t have that effect on me.
You know when babies move inside the womb? That’s exactly what it felt like to me. It was the only thing about pregnancy that I enjoyed. It’s that feeling where a car goes over a dip. (I used to love roller coasters too.)