I experienced a bug where the deck order was wrong. After one mulligan I put a card (a card which only one-of was in the deck) on the bottom of the library. No deck searching occurred in the match. However, I still drew that card after several turns. I check again after the match to confirm - and there was only one in the deck.

This appears to be an error in the deck ordering on the server side - I have attached the .log but this doesn't seem to be a client side issue.
I haven't been in many drafts and the ones I was in rarely had mulligans. Shouldn't be too difficult for you to find if you keep server-side game logs - happened on 5/29/2025 later in the evening (EST).

Spent 1k gems but likely would have still lost - more concerning that it calls the actual deck order and remaining gameplay into question.

Most likely this is in your deck order optimization routines - the ones that smooth gameplay by optimizing draws for traditional gameplay rather than full-random ordering. If I had to guess the weighting likely applies to each card in the deck and is triggered after the first hand is drawn - when it should occur after the final mulligan or keep action is taken.