Pokémon Sleep is investigating an issue affecting the Mew feature during Snack Time, with some players unable to claim the Eureka Seed (Mew) item under certain conditions. The bug is currently being investigated, and affected players will receive a gift in their in-game Gift Boxes at a later date as compensation. Pokémon Sleep’s developers apologized for the inconvenience and said they hope players will continue enjoying the game.
Source: Official Pokémon Sleep news