First, in case you missed it, Google announced that the Works with Nest API is being shut down on September 1 of this year, at which time the Nest binding will no longer work.
Second, different technologies and even different devices within one technology handle motion detection different. Some will sendCommand ON when ever it sees motion, usually with a cool down period where it goes blind but then never send OFF. Others will send ON every time it sees motion and then send OFF after a certain amount of time of no motion. Some will send ON every time it sees motion followed by an OFF and then a new ON command again for the next motion detection.
I don’t know how the Nest works in this regard but suspect it’s the first way. It send’s ON but never OFF. So any Rules that need to be triggered by motion needs to look for ON commands and not really pay attention to the state of the Item.