Read on below to learn more about a Pokémon Sleep bug notice:

Thank you for playing Pokémon Sleep!
The game is currently experiencing the following issues.
Bug Contents
Chansey with 2,000 hours of shared sleep time and a Good-Night Ribbon has higher helping stats than Blissey.
Status
This will remain in the game as is and will not be fixed.
We hope you continue to enjoy Pokémon Sleep!
Source: Official Pokémon Sleep news
Source: Pokemon