Said glitch being on a sidequest-required 'mon, of course.
http://www.nintendoworldreport.com/news/59602/pokemon-legends-arceus-receives-v102-patch-which-resolves-pokemon-catching-glitch
UPDATE 7:30am ET Feb 9: The patch notes have been released, including resolving issues with satchel collection offline and some other progression issues. Original story follows.
A mandatory update for Pokemon Legends Arceus has fixed a bug that made a sidequest more difficult.
Although the full patch notes have not been published yet, an issue observed in the launch versions of the game has been fixed. The Pokemon Cherrim - who has the innate ability to change forms in sunny weather - could not be caught if the ability was active.
The glitch was especially notable as one of the requirements for a sidequest (#17 "To Bloom or Not to Bloom") was a completed Pokedex for Cherrim, and a common technique for catching one required sunny weather active.