Conducted some serious investigation.
The committee consisting of me and marcan looked through many sources, including the JP broadcast, into which the artifact was still present, and hereby conclude with a verdict of "it was made by the voice actress through the formation of a transient whistle, likely between the teeth and lips".

The resampling artifact theoryis ruled out due to it not presenting as such, and the theory it was an encoder error is also ruled out since the rest of the band looks normal and not energy-starved.
The encoder carried out its duties as directed and kept this during encoding.