There’s a LOT of conflicting information out there. I heard originally that if Gen 3 roamers use a move to end the battle, that they will despawn for good. But a while ago, I saw a video that I cannot recall, where someone say it is possible to respawn them, but I can’t remember the details. I believe it was from a well-informed FRLG RNG manipulator, but I’m not sure.
Before answering, please ensure you have up to date information from a reliable source explaining it.
Thank you