As far as I remember, my Apple Watch Ultra 2 never stopped at 80%, although the Optimized Charging Limit is enabled. I'm only charging my watch every 2-3days when it reaches 20% or less. And my watch somehow learned, that I need it to be charged up to 100%
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Your Apple Watch takes time to learn your charging patterns before it uses Optimized Battery Charging or Optimized Charge Limit. If your watch is new, the charging screen will say Charging to full. Optimized Charge Limit changes dynamically depending on how you use your watch. Based on your usage patterns, your watch might charge to 100% on some days, and less on others.
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copied from: About Optimized Battery Charging on Apple Watch - Apple Support