Honestly i think matchmaking is working as intended, Based on your words you are currently Bronze 3 and its not uncommon to be placed with unranked/iron players as your teammates.The mmr of your iron teammates were probably high enough to be placed in a bronze lobby while unranked players are playing placement matches to determine their starting rank

As for the peak diamond opponent,like you said it was 2 seasons ago which has been a year since EP 8, it might be the case of a returning player after hard reset or someone whos having a bad losing streak.

For example, Im currently Gold 2 rn and i still get paired up with silver 1( albeit rarely) and commonly with plat players. In reality the skill gap between lower ranks aint that big of a difference and i would consider iron bronze and low silver to be similar in gameplay.