I think SY is decent, I don't think Alex's vocals are the problem at all.
The issue is that the song (and the EP in general) was not mixed properly, so the mix sounds flat and too rough for my liking. Obviously it was a free release, but she should know better than that. The whole EP didn't sound great at all and her vocals sounded off as a result, but I like her delivery on SY.
Syco turned her into a clean cut popstar so maybe some aren't used to this more sultry side, but as also seen on BTS, I think she can definitely pull it off.