I really like it too and the choreo looks nice, composition wise it's right up my alley and I think Yoona's low note is the killing part.
I sorta get some of the criticism and for me it's the celebration part in the chorus, we're a bit saturated with celebrations and they hit different. If you include the B-sides though the album is super varied. I think Heavy Serenade was Nmixx's best album to date with Blue Valentine being their best TT so it was always going to be hard to top the last two comebacks but overall Strange Muse is really good.
I would rate Birthday Wish as my 3rd, 4th or 5th favorite title track which is not bad, but it's going to take a few more listens to know for sure!