Sounds like another case of the no good diaphragm on the carburetor's vacuum piston.
Sparker, looks like these carburetors have a (rubber / rubberish) diaphragm that deteriorates over time and doesn't seal properly. And when that happens, it doesn't allow enough pressure difference on either side of the diaphragm to cause the vacuum piston to slide all the way up in the carburetor when needed. Therefore, the engine doesn't get as much airflow as is possible. In essence the airflow is choked off, but the air fuel ratio is good because that piston is also controlling the jet needle within the needle jet holder and main jet, so the bike runs good at it's new airflow limited speed.
That's what I'm guessing here...
2016 Honda Rebel 250 - The "Piglet."
AFR sensor equipped and downsized to a 0.105" main jet.
The only changes so far.
Bought on 6/29/19 with 44 miles.