Earth Power changes into Hyper Beam after a while.

[Syn]

So i used the move tutor in Rezzai Cavern, Which teaches Earth Power, and it works just fine, But after a while the move changes into Hyper Beam, with no interaction by my side, just randomly changes.

[Gamsrfun]

How long is the while? Is it after, like a day? If it is, as long as the tutor isn't at the bottom of the cavern or something, just go back and relearn it, getting rid of hyper beam. If it's every hour or 10 minutes or something, then idk.