Responses
I think Masato just forgave him right away and considered it as "not a big deal" considering how Masahi was busy. But he might have been quite hurt, and we might get a flashback of it. Because even if you dont csre about your birthday, someome who forget it implies that for them its not important either. And thats whats painful.
But its also possible that Masato didnt care at all as you said.
Let’s remember masato forgot his own birthday as well. I don’t think he really cared/care that asahi hasn’t noticed it either because masato has been more worried about asahi and trying to get him to open up more .