This is purely anecdotal, but it very much seems like any time I duo the scales are tipped against me with matchmaking. Going to be tracking duo vs solo win rates over the coming weeks and updating this post. Seems to be on any account I have across a variety of ranks.
Very unfortunate this seems to be the case. My friends and I usually just sit in discord and talk to each while playing solo queue games (even though we are around same ranks as each other, within 1-2 divisions). Riot seems to not want ppl to play together.