What is so hard to understand in that the the two instances are entirely different?
Case of Fake Itsuki: Nothing shows that Fuutarou views the quints as love interests, so Miku wants to end their partnership. Fuutarou already stopped coming to their home, even when Miku said he could have just hang out with them. If he is no longer needed as a tutor, then maybe he can notice them more as girls.
End of the case: Itsuki agrees with Fake Itsuki, she tells Fuutarou they are pretty much friends now, not partners. Miku apologize for using Itsuki's identity to change their status quo. And what was that made Fuutarou the most annoyed with himself? The fact that he couldn't guess who was Fake Itsuki, so he even refused to let Miku tell him. The issue with the partnership was just an afterthought.
Case of Fake Miku: Ichika using the identity of one of her love rivals', tells via said rival that Ichika loves Fuutarou, and said rival is supporting them, indicating the rival has no feelings for Fuutarou.
So how on Earth did Miku's actions screw with any of the quints' chances in the Fuutaroubowl compared to Ichika's?