I agree. I mean who wouldn't be frustrated about it, he put so much effort in showing him again and again how much he cherishes him, to make him feel at ease bcs he went through a bad breakup before, he is also dealing with a lot more compared to Garam - his mom cutting him off from money, finding and starting a new job, and dealing with that trial with Garam's ex. For Garam to use Jae being busy as an excuse for not telling him all this time about his birthday.. idk I feel like that would suck for anyone who truly cares about that person, and also Garam's reaction to the rings that was a really bad reaction, he seemed upset about the rings visibly, so who wouldn't get hurt, I mean even if you are not sure about the commitment (it's not like Jae proposed but ok) just for the sake of the other person you would try to have a better reaction or try to explain yourself better... I think it's all the little things that happened over the course of their relationship where Garam was not being fully honest piled up and this was the last straw for Jae. Just my thoughts tho

I kind of understand garam's feelings about the ring as he also bought rings for them a few chapters ago but didn't mention it bc of the pressure of the family situation. It's possible that he's still hung up over the words jae's mom said and doesn't want to do something that would put more pressure into their relationship as they already have problems with the apartment and his parents.
What I don't understand is his reaction. It feels a little uncharacteristic for him. He didn't even feel moved at all, he looked really uncomfortable for a person that had the same thought about buying something as significant as a ring. It's really weird bc we already went through the whole tsundere phase so I don't get it.
That's why I think it's totally valid that Jae had that reaction given the circumstances. He's been frustrated for garam's lack of communication through the whole relationship so it was a matter of time for Jae to have a crash out about it.
I just hope that they talk it out for real this time and don't resort to sex again.
I really love them so I'm hoping for the best ╥﹏╥