This is confusing as heck.
At first I don't even know whose flash back that was.
After looking at it carefully, turns out that was Miyu's flash back since the 'wh-why?' part is when they first meet. Then on re-read, it turns out the other panel is from the same event, but the dialog was changed.
During the past chapter, he said something along the line of 'I remember you now, you're Tahara Yuushin right?', but in this chapter, the translation is a bit confusing when they changed the dialog and use 'the person close to you is Tahara Yuushin' which I, and probably a lot of people, would interpret as, 'You and Yuushin are actually close', but when we take a look at the past chapter, you'll realize it actually meant to say 'This person next to you is Tahara Yuushin'.
Now, what I think the author was trying to hint from all these and what Miyu realized is,
This Hongo is not the same Hongo the real Miyu was close with back in the day.
That's why he didn't remember Yuushin right from the get go.
Probably the Hongo everyone's know was the one that breaks down and become the neet/jerk. This one is the brother that has to now live as Hongo Haruto and has to learn every aspects of the real Haruto to live as him; so he didn't really know Yushin or Miyu personally from the beginning and only see pictures and memorize just a few key person (like Miyu), so he can't remember Yuushin right away and has to connect the dot from Miyu to Yuushin. Also Yuushin had a throwaway line in past chapter questioning can someone grow that tall in the span of time they don't meet.
All these, to me, points to this is the brother, not actual Haruto.
Some people mentioned he's a she...
yeah, maybe not even the brother, but the sister
EDIT
Yeah... when you look at it through this lens of he's actually the sister living under the identity of Haruto, everything seems to click into place.
- the fact he notice facial features difference from Miyu
- the fact he doesn't give a second though to staying over alone with Miyu
- the fact he blushes when Yuushin asking him to run away together (no proof, but I bet this sentence doubles as 'let's elope' in original JP)
- the fact author draws him with long lashes at close up (Yuushin doesn't have this feature, only Miyu)
We might even find difference in some facial features (such as the two moles under eyes) if we really dig to find a picture to compare.
I kinda believe in this theory more and more.