Pokémon GO developer Niantic has confirmed the existence of issues occurring when encountering a Pokémon that you have previously fed a Berry to. Therefore, the developer has extended the deadline to claim research tasks to August 27 in order to resolve this issue. The official statement from the development team is as follows:
Encountering a Berry-fed Pokémon causes app to become unresponsive
We are aware of an issue on Android version 0.111.4 and iOS version 1.81.4 that causes Pokémon GO to become unresponsive when encountering a Pokémon that you have previously fed a Berry to. We are currently working on a fix but in the meantime, you will need to restart the app to continue playing.