For years I have seen my voltage drop at high RPM from 13.9 to 12.2 when over 6000 RPM using Autozone and other reman alternators.
Info from a local electrical rebuild shop said that our alts have a built-in provision which lessens voltage at higher RPM's.
I then examined my current alt, and found it had a pulley which is smaller than stock?
Having some stock alt cores, I installed the larger stock pulley which resulted in less alt RPM, and more stable voltage.