I just noticed this the other day, but my Apple Watch 4 is always "offline" when I open the Find My Phone app...It's connected to my phone and is working fine, no phone connection issues. As a matter of fact, if I open Find My Phone, and then tap on the watch listing...I can even do the "play sound" and my watch will play the sound...until I dismiss it from the watch. But like I mentioned, it's always "offline" in the app...with no location shown at all...My phone, and iPad are both listed, and showing their locations fine...