Damn Ichika, that was low... she just dropped to the bottom for me, hurting Miku like that smh, Miku will always be best girl!
This manga is really leaving me an emotional mess, wish I didn't catch up I need more!!!!
I'm with the side that thinks Fuutarou recognized Ichika under the disguise. Mainly because I don't remember Miku telling him about Ichika's movie like he said Miku did the day before.
Methinks Fuutarou wasn't sure if she was really Miku and he didn't want to make a mistake like he did in the Scrambled Eggs story arc. so he told her something only the real Miku would know is a lie. Given that "Miku" played along with it, then he deduced that she wasn't the real one.
Now that he actually took a good look at her (notice that he wasn't really looking at her until the last two pages) and considering that Ichika showed him her props to disguise herself as one of her sisters just that morning, plus the whole hype among their classmates about her movie role, I'm guessing he won't have much trouble deducing that it's Ichika trying to pull his leg. Probably he will get a tad angry rather than just annoyed at her "joke" given that it was a "fake" confession and he isn't taking love matters as lightly as he did before.