@JWM The #1 reason for range just stopping at a specific note is too much damn pressure.
A buzz is nothing more than the lips opening and closing at a certain frequency. If we apply large amounts of arm pressure to the embouchure, we need far more air pressure to blow the lips apart. That is not a very responsive system!
In my view, the easiest way to break this habit is to turn the testosterone down. Play ONLY very, very softly for a couple of weeks and focus on the air doing the work. Overdose on pianissimo lipslurs and long tones. Practice to get the sound to start on a wisp of air.
Generally, it takes only a lesson or two to get students doing this reliably. The biggest problem with a social media recommendation is that body use can not be controlled. If someones body is twisted into knots, a big relaxed breath is a challenging thing. Exhaling also becomes a real chore.
My full view is in several posts that I made called "the circle of breath". This is a collection of things from many disciplines. Google it and if there is something unclear, just ask.