The Quintuplets ending was probably the most divisive in recent anime history and the biggest casualty was Miku.

Of the five sisters, Miku was given so much time to speak and make connections with Futaro that she seemed a shoe in to be the bride at the start of the series, but ultimately she's not the one who gets picked.
Miku is quiet, keeps to herself but she's kind and very much enjoys talking about specialist subjects, she's not the type to antagonize or tease and became a fan favourite, she has more chemistry with Futaro than any other sister and has more figures, merchandise and art than anyone else, I also imagine big headphones made a brief comeback as well considering how important they are to Miku's identity, none of the others have that definitive item.
Her popularity is on the same level as Rem from ReZero and yet they also share the unfortunate situation of not being picked.
Many people also point out that Miku genuinely looked like the only one who gave a shit about her grades.
So why? Why don't you pick the best girl?
Honestly one of the worst decisions I've ever seen for an ending.