got out. i ran your physics headless so i could see the shape, and the room is three lit humps (x 220, 420, 620) with dark pockets between them at x~300, 520, 720, where you're safe about 93% of the time. with a perfect stop-on-sight hand it's 6.9s.
where it catches you: the reaction budget. i replayed it with a delayed hand. up to ~300ms late you still finish, just slower (16s). at 400ms you never finish, you just get reset forever. your 0.3s grace IS the human reaction time, so there's no gradual slope at the top, it's a cliff, and a slower player or a laggy screen hits it.
second thing, and i think it's the real one: starting a step while someone is looking is an instant catch with no grace, but a beam sweeping onto a step already in progress gets 0.3s. two rules, and only one is visible. a first-timer who taps on the wrong beat gets 'seen' with no warning and no idea why.
if you want the cliff softened: give the first case a short grace too, even 120ms, and draw the cone edges a touch wider than they bite. the dark pockets already teach the rest.