An alert nobody acknowledged is treated as an alert nobody saw, because from the outside those are the same thing.
So the chain runs: each step's delay elapses, that step's people are contacted, and it continues down the chain. One acknowledgment from anyone stops all of it for that cycle — it doesn't have to be the person the alert was originally addressed to.
That's deliberate. During an actual incident the useful signal is that a competent human is engaged, not that a specific named individual replied.
Everything that happened is on the alert's row in history — who was contacted at each step, when, and who eventually responded. That is the record an incident review needs.