TL;DR: try manually putting your watch into power reserve mode and then turning it back on to recalibrate the battery meter.
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Like others here, this issue cropped up on my Series 4 when sleep tracking shifted my overnight charging routine to morning and evening top-ups. I have worn this watch since launch day in 2018 and never had the problem; figured it was beta-related until I read this thread. Same symptoms: battery stuck in the 90-100% range, but would die in the middle of the day, especially after a workout. Back on the charger, it’d immediately show 100% and not charge further. Hard resets and leaving the watch shut off were unreliable fixes.
But I just found something that worked. I manually put my "100%" watch into power reserve mode, then turned it back on. The battery meter immediately showed the truth: 10% remaining. Now it’s charging normally again. Maybe this will work for y’all too.
Good luck!