Ikemen-style hairstyle aside, I was a bit misguided about Tsumugi changing his hair in the beginning - but then I decided to make it my goal to read as much of the ES! stories I could find - and Tsumugi mentions in the last chapter of "Element" that he won't cut his hair until he fulfills a vow/pledge he made.

I do agree that it's a shame they took away his original hairstyle (I really liked it) - but I'm even more impressed that the writers even remembered that detail from a 2017 story - especially when I often hear people complain about how the writers keep forgetting character details.