In the past, webhooks did not respect the "when down, alert after" threshold for Uptime checks. About 2 years ago, this feature was implemented and is enabled by default for new accounts.
We recently observed this behavior, contacted support, and had the feature enabled.
It's current state, as well as the ability to turn the feature on/off, is only available to support.
It would be helpful if this feature were controllable by the customer via the web UI.
Thanks!