v188.8.131.52 Build 9198.1663030
Yuan shao after reaching the rank of marquis (which allows him to vassalize other factions) begins extremely quickly vassalizing factions, especially weaker ones. But also within my campaign he vassalized people like Sun Jian (who was in a 3 people coalition) aswell as his coalition members, and also kongrong (who was in my coalition). He even offered me an option to vassalize despite me owning same amount of regions as he does.
To reproduce it i think all you need to do is wait for yuan shao to get to marquis rank.