And what issue would that be?

Loads of
mv: will not overwrite just-created [file] with [file].
errors during make install (basically one for every file).

Try upgrading fileutils to the version in the rawhide tree and rebuilding mx 
to see for yourself.

Might still be a fileutils issue; I'll debug this once the S390 stuff is done, 
unless you beat me to it.

I was thinking of what the POSIX compliance issue would be... I've never seen
Posix demand "don't do what the user say" before.

Suspicious items from the changelog:

* mv now prompts before overwriting an existing, unwritable destination file
    when stdin is a tty, unless --force (-f) is specified, as per POSIX.
    [doesn't state what it does when stdin isn't a tty]
* cp's -P option now means the same as --no-dereference, per POSIX.
    Use --parents to get the old meaning.

Is stdin really a tty when using rpm to build a package?

rpm does not change stdin, only dup's fd #0 for running scripts. So stdin
is whatever it is/was when rpm is/was invoked.

The error seems genuine; just looked through the cp code causing the message:

              /* Don't let the user destroy their data, even if they try hard:
                 This mv command must fail (likewise for cp):
                   rm -rf a b c; mkdir a b c; touch a/f b/f; mv a/f b/f c
                 Otherwise, the contents of b/f would be lost.
                 In the case of `cp', b/f would be lost if the user simulated
                 a move using cp and rm.
                 Note that it works fine if you use --backup=numbered.  */
              if (command_line_arg
                  && x->backup_type != numbered
                  && seen_dest (dst_path, dst_sb))
                {
                  error (0, 0,
                         _("will not overwrite just-created %s with %s"),
                         quote_n (0, dst_path), quote_n (1, src_path));
                  return 1;
                }

So this is a fileutils bug? It certainly looks like one...

Reassigning component.

Fixed in 4.1.2-2.