@false ok that’s weird then. Supposing you’ve done a hard reset after the update (was necessary with my S9PP!), i also don’t know what could be the issue 😕
You could power off the watch between your workouts and see if the battery still drains. If yes, its a hardware issue (warranty repair?). If no, software problem and thus there might be a fix (reinstall update, bug fix with next update, etc)