I would say that RK's reaction was consistent with how his character has been for few months now. He has big problem in trusting people no matter how much he or they love him. He can't take Madhu's (or anybody else for that matter) statement at face value.
His reaction in yesterday's episode was the result of his deep founded insecurities. Insecurities about Madhu leaving him for someone else. He has hard time believing if someone can truly love him. Even before the party he told Madhu "How did I got you" or something like that
So when he saw sultan he was again taken over by the insecurities and we know what happened after words. I'm not really justifying his deeds but trying to think why he has done it in the first place 🤔
Btw I felt so sorry for Madhu. She had to under go another humiliation even worse than last time because her character itself was questioned this time