A couple of observations I've noticed while looking into the issue.
1. The charger will consumes .1 watt without the Apple Watch on it.
2. The Apple Watch charges at 1.7 watt. The watch will be warm.
3. The 1.7 watt charge ONLY occurs while the screen is off. Tapping the counter to turn on the screen (nightstand mode) drops this power to .6 watt and wont go back to 1.7 watt until the screen falls asleep again.
4. At ~95% charge state the power drops to .6 watt regardless of screen on or off. At that point charging dramatically slows.
5. .6 watt remains even hours (2+) after the watch has reached 100%. The watch will remain warm.
6. After observation 5 the charger finally drops to .1 watt, percentage will be at 100% and the watch will cool off (colder than skin temp). Remember this .1 watt is what the charger uses so at this point the watch is not charging at all. However the battery widget in iOS does still show the watch charging so it knows it’s on the charger.
The above was observed over a couple nights of it charging properly to 100%. I’ll need it to only charge to 95% again to see what is changing.
If I were to guess I wouldn’t be surprised if it got to 100%, stopped charging and didn’t start again until 95%. At that point it would charge at .6 watt (slowly) and get warm. I could be wrong but in recent memory I feel I notice this more often after the watch has been on charge longer than normal like if I take it off at 6pm the night before.
Without a way to log charging this will tough to know exactly though.