The Bloomburrow rare-to-mythic upgrade rates at https://magic.wizards.com/en/mtgarena/drop-rates have not been updated. I put this direct to customer support since I suspect it'll get buried here, but my understanding is that this kind of transparency has something to do with legal requirements. I'm not particularly bothered since I only use if for a draft calculator, but someone might want to look into posting that info since it's been about a month. It looks like everything's good up through Modern Horizons 3 though, so perhaps the info has been moved to somewhere else?