Actually the opposite for me, 6.01 and 13.1.3, I used to get 2 days of battery on my watch (usually around 60% at end of day) but the last 3 months or so, always on the latest OS, usually at around 50% at the end of day so I charged it every night.
3 days ago it was 58% and ended the second day at 5%, last night at 55%.
So improvement for me as there is no change in usage pattern.