Sierra Neck / Fretboard Curve

[Mr Bassman]

I have a Sierra U12 on loan/trial.
I've noticed that the neck isn't completely flat. It's level from the nut/rollers up until around the 20th fret, then it slopes down by about 3/16" towards the changer.
It doesn't look like a warp as the mica is intact (apart from a small piece which has been refixed possibly due to damage)
Any other Sierra owners noticed this?