As one of the people who use subject case after
but, I can say that I at least think of
but more as a conjunction than as a preposition. As such, the subject becomes whatever comes after
but and agrees with that “Nobody but I am going to tell you this”, for example). Switching to an unambiguous preposition,
except, I also switch to oblique case and no agreement with the ‘second subject’: “Nobody except me is going to tell you this”. —
Janus Bahs Jacquet Jul 3 at 23:23