autofs(8) manpage says:
After   the
       auto.master configuration file  is  processed  the
autofs
       script  will  check  for an NIS map with the same
name. If
       such a map exists then that map will be processed
in  the
       same  way as the auto.master map. The NIS map will be
pro-
       cessed last.

But that's not true, the autofs script only looks at the
auto.master map if /etc/auto.master does not exist.

The autofs-master-yp.patch patch included with the autofs
SRPM mentions this behavior.

I suggest that the behavior be modified to be compatible
with Sun's automounter, checking for a line starting with
"+" and including that map (e.g. "+auto.master"). It is
useful to be able to have both local automounter
configuration and to be able to pull the configuration from
NIS. This would be a trivial change to the autofs script.