Yeah I understand why you gave them iframes, but the ambiguity makes it very frustrating. Having the enemies jump out of the background (being greyed out until they're hittable) would be the best choice in this case. The jumping felt finnicky specifically because of either the movement's inertia, or the way the jump works (it has a preset timer/arc after you release the button maybe?), the exact problem is hard to pinpoint but basically it felt like I would often slide on landing, removing a sense of control. A lot of that is psychological, but important.