When a developer who wrote the relevant code says that there was never a particular piece of logic in that code, so nothing could have possibly gotten removed, since it wasn't there in the first place --
-- it's not an illusion either.
But "something", "somewhere" could have changed.
Tales from support (tm):
Just recently someone's *all* contacts became unavailable to Aqua Mail's autocompletion -- "after the new version update" of course
And then this user went into Google Contacts on the web, and discovered that somehow all his contact entries got removed from the "My Contacts" group. He didn't do it, what happened? We'll never know.
He put the entries back and his autocompletion began to work again. Had nothing to with Aqua Mail.