I'm not entirely sure if this was caused by the slow falling potion or being near the dragon at the portal. I tested this in a creative world and couldn't get the potion to fly off again. I tried both slow falling and being near the dragon.
When you throw a potion straight up, it flies very far up into the sky, out of distance.
When an entity gets damaged within the dragon's hitbox it gets launched super high. That applied for the splash potion probably?