2022 Yamaha JetBlaster suddenly won’t rev past 6,000 RPM after purchase—55 MPH before, now only 40 MPH

I bought a 2022 Yamaha JetBlaster last week. During the test ride, it hit 55 mph without any issues. This week I took it out with my girlfriend, and together we weigh about 315 lbs. At full throttle, it would only reach about 6,000-6,100 RPM and around 38-40 mph.
At first I assumed it was just the extra weight, but after looking into it, it seems like the RPM should still be much higher even with a passenger. The engine starts easily, idles fine, and otherwise runs smoothly—no obvious misfires, hesitation, or strange noises.
Any ideas what could cause it to suddenly be limited to around 6,000 RPM? I was thinking maybe worn spark plugs or possibly too much oil in the engine, but I’m not sure. It ran perfectly during the test ride just a week ago.
Anything else I should check before I start replacing parts?