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I had the phone disconnect notification turned off (as discussed in this thread) on my Galaxy Watch 4 (LTE, 44mm). But in the last couple of weeks the watch has started notifying me whenever it disconnects. I've flipped notifications on and off again, hoping to fix this. But it doesn't change the behavior.
Also, those notifications don't appear in the history of notifications I can see on the phone. So it's possible that these aren't only from phone disconnects. But I can reproduce this by turning bluetooth on the phone off. So I know that's at least 1 of the sources of these.