I'm not saying this is what happened, but here is a legitimate case of why this could be happening: Players A and B are gold and silver respectively, they are friends and party up into Ranked. They get matchmade as gold. Player C matchmakes into their party, and is Plat. Plat players can and do play with Golds. After the match if Player C tries to join a party with Players A and B, we do not allow that because Plat can only play with 1 Tier difference away.

Why is there a 1 Tier difference max then? Mainly this is an attempt to stop boosters in Ranked.