It is by design yeah, and I do believe it that it can certainly be frustrating for the uninitiated. I took the From games as my rough blueprint, and the Heavy roll in Elden Ring is about as long as mine. If the player is able to zip around dodging everything, then the boss quickly becomes pretty harmless (unless you resort to them doing looong fake-out wind-ups before actually swinging, but that has its own frustrations). Like the rest of this genre, I wanted the player to have to learn the boss, time their attacks and dodges carefully and not just button mash their way through it.