Thanks, everyone, for your attention and support.
I already reset but the problem continues. The previous onwer is a friend of mine who has never noticed this behaviour.
He was kind enough to accept the return and will look for Korg assistance.
I'll post here the outcome. Thank you.
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Yamaha Tyros 5-76, Yamaha Motif XF6
Happiness is not only your right. It is your duty to be happy!