By manually moving the constants from .const to .data, I got this program:

       program main
       integer m,n

       call corrupt(7)

       write (*,*), 7
       stop
       end program main

       subroutine corrupt (a)
       integer a
       a = 4
       return
       end

to output 4. Thanks for pointing this out, 'concede_pluto. Pretty weird!