100% reproducible, most speed buffs do not affect you character’s forward speed when you jump. How to reproduce: drink a speed pot in the keep and race around. Then drink another one and go around the same route but constantly jumping. Compare distance travelled.
For example, Slayer’s movement speed ult talent works the same way.
It sure looks stupid and defies basic conservation of momentum, but I don’t really know. It’s also inconvenient at times since you just have to jump all over these maps because they are full of tiny obstacles.