When chains break, it is predetermined that the chain will do so before the Pokemon is encountered. You can see this on the PokeRadar app on the Poketch because as soon as you enter the patch of grass, your chain will disappear from the app.
Therefore, the Pokemon you encounter when the chain breaks is completely random and can be the Pokemon you were previously chaining.
Answering a question asked online 15 years ago is kind of nuts
