Bronze 2 Misha Demidov duoing with you is part of this issue. When you duo they Add Extra MMR to the match average for your team to account for premade synergy. Duoing is poorly implemented by riot as far as giving MM room to build fair games.
As in Mishas history they have 8 games with you I am fairly confidant in the claim you two are duoing that game.
The gold D4 player is at silver MMR going off his match average, not just your game but overall that's his trend. Riot has it set so demoting a Tier is massively longer gap then demoting a division, Last I checked it had to be what I assume represents an entire class interval. (drop into division 5 MMR prior to being demoted.) And they also give you a warning message prior.
In prior seasons youd see some folks near that range on the verge of demotion. Its not so cut and dry This year. There is Positional MM where your MMR is adjusted by role. Riot got rid of Positional ques but never removed positional MM. So you see a spike in tier average change from the Gold players Main role to one he is shit at + there a solid 6 game loss streak eating into his MMR balance prior to your match.
The gold player is Silver every other year, low silver I might add. So odds are they got a bit of luck and also some of this seasons Ranked Snafu gave them a leg into gold mmr but now their inability to play at that level has sunk them back to the MMR range they operated at for the last 5 years.
This stuff is really easy to spot when you just study up on MMR systems buddy.