This is not Yuuki Yuuna, but rather an original Yuuna.

The image date on Pixiv is February 17, which, as far as I could look up, is soon after Dengeki scans about Akamine Yuuna were leaked. The image is also tagged ゆゆゆい on Pixiv.

The idea here seems to be that, with the reveal of another Yuuna, this artist decided to draw an original Yuuna. Or drew her right before Akamine's reveal and was surprised when Dengeki leaked.

Maybe we'll see more original Yuunas in the future. There's potential for that, as the name Yuuna canonically has significance in the Yuusha de Aru universe.