I actually think it's justifiable because in the first place, I don't think Heejae managed to have a deeper connection with his brother and everything else because of his amnesia. Besides, he was treated badly after that too so I wasn't a little bit surprised that he was passive about everything. You also have to take note of the fact that, he bonded more with Taekhyun despite it being just a set up. He wasn't anything else when he forgot everything but with Taekyung, he had the liberty of finally feeling like a normal person. If you've forgotten as well, didn't he try to take advantage of the company as well? Which allowed this kind of situation in the end. It's not that he isn't angry enough but rather, he was detached from how things have been going until Taekyung came back to his life which lead to him nothing but Taekyung by his side to latch on since Taekyung's the one who showed him life after he supposedly lost a part of him.
it will forever bother me that heejae was no angry enough at taekyung