Perfect pitch does not always remain perfect. There are studies and anecdotal discussions observing this phenomenon. For example:
Personally, I know two musicians whose perfect pitch has changed: one a working professional pianist in his 70s (as of this post); the other an amateur multi-instrumentalist in his 40s (again, as of this post). In both cases, they described difficulty playing, because the pitches coming out of their instruments no longer matched what their inner ear expected. (The instruments were in tune, of course.)
From @Richard in his answer to Unlearned Perfect Pitch / never had it?:
To quote Gary Karpinski in his Aural Skills Acquisition:...
Finally, it seems that many with AP find that at some time during midlife their perception of AP begins to "shift," eventually mapping once-learned pitch-class names onto incorrect pitches (Vernon 1977; Ward 1999, 280–81).
In a footnote, he mentions that this shift often occurs in the sharp direction, so that A440 will start to sound like B♭. Furthermore, this shift seems to stretch with age: the older someone is, the wider (and more "off") their AP may become.