Do you know the radius of the fingerboard?
Lots of vintage guitars, with a small radius (7.5 - 9.5") will "fret out" on bent notes, esp. above the 12th fret. That is whay newer guitars have a larger radius (12") or a compound radius (12-16"). That gives you a flatter fretboard overall and with a compouond radius, it gets even flatter as you move up the neck. Though, first position chords are more comfortable on a smaller radius neck.
If I had to bet, I would guess that one of the higher frets was not properly dressed at the factory, or was not fully seated. If you see a fret (above the 18th) with a tiny visible gap between it and the board, you could try tapping it gently with a plastic headed hammer to seat it. If you sight down the neck, from the bridge end, and see a fret that is too high (proud), you can block sand it with 320, 400 & 600. That will make the top flat, so you'll need to re-round it with fine sand paper or synthetic steel wool without out a block.