Same here - it didn't feel quite right（￣へ￣）
I think it's because it was a rather quick revelation and we are never privy to Jun's thoughts or his backstory. We are expected to take his word for it that he has resolved his feelings for Ryuu and has built feelings for Takumi. Given that there is no evidence that he feels otherwise and there is evidence that he does feel this way - his jealousy over his college buddy's prank and his eagerness to explain their relationship to Ryuu - there's no cause to doubt him or the mangaka who wrote him. At least, not for me. However, I can understand the feeling that there should have been more.
Is it weird that I still feel that the seme did not really .... see the uke as he is? like still not there yet....