I discovered the challenge of getting clear notes above the 18th fret when recording and having muted notes, the wrong notes and even no notes just the percussive sound of the string hitting the next higher fret. One thing that affects the sound is the challenge of getting the left hand fingers between the frets because the frets are so close together. Sometimes I have to turn my finger sideways so that the finger will fit between the frets. ( Almost at a 90 degree angle where my left hand fingernail is almost noting the string). Ultimately you are listening as you play for clarity and adjust your left hand fingers to whatever position produces a clear note.
Clearance is another issue. To see this, note the 1st string at the 18th fret and notice how close the string is to the 19th fret. It barely clears the 19th fret. Now note the 1st string at the first fret and notice that there is a lot more clearance above the 2nd fret. So when playing above the 18th fret you have to play the strings lighter so they do not hit the next fret as they vibrate.
So to get clear notes above the 18th fret your banjo has to be adjusted properly where the frets are at the perfect height relative to each other. ( Check with your repairman).Your touch has to be perfect so the strings do not vibrate any more than is necessary which requires listening very carefully. Typically when you play high up the neck the right hand is playing near the body of the banjo. To test this note a fret above the 18th fret on the first string, then move your right hand in 1/8th inch increments and listen to how the tone and power of the banjo changes.
One last thing, you will notice that the volume of the notes played above the 18th fret using half the force as playing towards the 1st fret are about the same.
Please share with me your thoughts and experience with playing above the 18th fret.