We've had a few instances where K-Pop groups have sung for anime OP's/ED's. TomorrowxTogether (TxT) did one for a Black Clover ED. The group is not too big, but not very small, either. If I had to guess, it'd be that they've been gaining popularity recently. I could certainly see that, with the release of 'Blue Hour.' Must be for more exposure, got a deal somewhere.
It's certainly not all too common, especially when not including the already groundbreaking manhwa adaptations. But with the rise of foreign relations/work force they have been slowly garnering for both the animation industry and in general society, maybe this is just the start... Of something that will quickly die. Who knows.