Blue Archive global has been hacked, with notice board vandalized and cafes filled with Koyuki. Also some players have spotted Hatsune Miku and like every students in the cafe.

Refer to the tweet in artist commentary for examples.

Whoever this hacker is, they got good taste in pranks.

Soalner said:

Whoever this hacker is, they got good taste in pranks.

The prime suspect.

No_clue said:

The prime suspect.

So... Koyuki decided to become Agent Smith as a prank?

Eboreg said:

So... Koyuki decided to become Agent Smith as a prank?

I wouldn't put it past her.

Considering the hacker's apology this theory becomes even more believable.

R.I.P. Nihaburner