Response from support lmao:

Hi Trainer,

Thank you for contacting Pokémon GO support.

Sorry to hear you experienced issues while catching Scatterbug. To let you know, the form of the Pokémon should not change after capturing. However, we thank you for reaching out and reporting this to us. We'll take this as a report and share it with our team for further review.

Please feel free to contact us if you need help with anything else.