No successful responses received before deadline

You don't have enough probes once the monitor is up for the Time Until Up timer to work.

K70501161:  Expected behavior of "Time Until Up" in monitor config

You have an Interval of 1 second if the member is down, but when you get a successful response the monitor switches to 28 seconds (Up Interval), and starts the Time Until Up counter.

You will get a successful probe at 28 seconds, but Time Until Up discards the last probe, so it does not see any successful probes, and stays DOWN.

In my opinion, an Interval of 1 second is too aggressive, and the Up Interval is too long (10, or 15 seconds maybe).