Why did Wang Junkai’s mouth bleed?

It should be that Sui Yu (played by Wang Yuan) misunderstood Xia Changan (played by Wang Junkai) and punched him, causing blood to flow. Did you notice that after being beaten, Xia Changan (played by Wang Junkai) said "We have never been friends", so Sui Yu (played by Wang Yuan) should have said something like "I treat you as a friend, how could you do this"? , but it’s just my guess