This issue can be solved by clearing the cache in your Pokémon HOME application. Please try the following steps and see if it works.
▼ How to clear the cache in Pokémon HOME
1. Open the application and tap the menu button (an icon with three lines) in the upper right corner of the title screen
2. Tap "Clear Cache" (the fifth option)
3. Tap "OK" when the message ending in "Would you like to try clearing the cache?" is displayed.
4. A message saying "Now clearing cache..." then, "Cache successfully cleared!" will be displayed. After that, please check your Pokédex.