Confusion around blue vs gold XXL badges with evolved pokemon

Hi all! So after a quick search on XXL Pokemon and corresponding badges, it seems like the gold just means it's the largest that you've ever found. However, once you evolve it, the badge seems to always turn blue. Is this a bug or am I missing something with these badges? Has anyone ever seen a gold badge on a Pokemon they've evolved?

Asking partially to see if I'm misunderstanding what the color differences mean.