Historic Brawl Alchemy Deckbuilding glitch
I'm currently unable to change out the old version of Goldspan Dragon with the Alchemy version in my Historic Brawl deck. It won't let me remove the card, I can add the Alchemy version and make it 101 cards for some reason which then makes it invalid.
Changing cosmetics on it doesn't work like suggested previously, nor does changing the format of the deck to one thing then back to another.
Had the same issue earlier with "Luminarch Aspirant".
But unlike what you're reporting, changing the deck temperorily to "Friendly Brawl" did work for me...
Having this exact same issue with multiple cards in multiple decks