devtrackers.gg
https://magic.wizards.com/en/news/mtg-arena/mtg-arena-state-game-the-brothers-war
"These can replace a common rarity card in MTG Arena Store packs—which means it is possible to get more than one rare or mythic rare in a pack."
The BRR Mystic Forge is indeed mythic, but your BRO rare can also potentially be mythic, and these two slots are rolled independently. What you're seeing in the packs screen during pack opening is that the highest rarity card is getting sorted up to that center "slot." You should still have a BRO rare in each pack.