Welcome Kymco Owners!
It sounds like a shift with the RPM sound going down. I'm sure its just the belt sliding.
You're sure it's something that doesn't happen? I don't know where you're getting your information, but the belt doesn't slide in the variator. If you're hearing a strange noise, something is wrong. If you keep riding it, you will figure out what it is fairly soon, if it's something wrong in the variator or clutch.
Yeah, well, it is hard to tell what is being described...