The Unown fled because you examined the campfire while catching it.
According to this Reddit post (so yes, the glitch is discovered) and experience from you, the other answerer SmellyJacket, and I, examining an interactable object or triggering a cutscene will cause the Unown to disappear after breaking out of its Pokéball.
The Unown will respawn when you go to Jubilife Village and back into the original area (in this case, the Crimson Mirelands).