This is not specific to MacVim, I get it also on Linux in gvim: it
happens because the "terminal" (or pseudo-terminal) in which Vim runs
any shell command is a "dumb" terminal, which, like a teletypewriter,
can go forward but not backward. Any curses-based program (such as
less, used normally by man to display its output) which expects to use
the terminal as a TV (moving about in any direction) rather than as a
teletypewriter, may get problems.
I'll try to forward your question and this reply to the vim_use group.
Best regards,
Tony.
--
Faith, n:
That quality which enables us to believe what we know to be
untrue.