The following issue occurred in the game.
Under the following conditions, received Eggs and Pokémon in the Day Care of Bellis’s Lab may disappear.
1. An account that has been linked to two or more devices is using both Ver. 1.10.5 and 1.11.0.
2. An Egg is received, or a Pokémon has been hatched from an Egg on a device using Ver. 1.11.0.
3. The game is played on a device using Ver. 1.10.5.
4. Eggs or Pokémon received under the second condition may disappear on devices using Ver. 1.11.0 if an Egg is received again, or a Pokémon is hatched from an Egg.
Note: You can check which version of the app you are using from the upper-left corner of the title screen.
June 24, 2020 at 11:00 p.m. – June 29, 2020 at 1:00 a.m.
Note: If the first three conditions were met during the occurrence period, this issue will still occur if the fourth condition is met after the occurrence period.
Upon receiving an inquiry, we will verify if any Eggs or Pokémon have been lost due to this issue, and send compensation to the affected players.
To send us an inquiry, please go to Other > Customer Support > Contact us from the Poryphone Menu.
To all players who have been enjoying the Eggs feature, we sincerely regret any inconvenience.
We hope you continue to enjoy Pokémon Masters.
Source: Official Pokémon Masters site