The buildroot in specfiles is ignored on purpose, NOTABUG.

If a package depends on an exact buildroot, basically the package is buggy, but
since historically rpm has honored spec provided buildroot the new behavior is
certain to "break" a bunch of packages. Just curious: what's the scenario in
your case that "requires" a specific buildroot?

I have always built in /usr/src/redhat.  When y'all changed the default to 
build in ~/rpmbuild, I redefined _topdir to /usr/src/redhat in my ~/.rpmmacros 
file to put back the default, but that doesn't change buildrootdir, hence I 
noticed the problem (since it tried to use ~/buildroot/BUILDROOT, which doesn't 
exist).

Right, in that case there is an actual bug involved: while the spec buildroot is
ignored on purpose, the default buildroot should follow _topdir unless
specifically overridden.

Fixed upstream

Fixed by rpm-4.5.90-0.git8444.1 in rawhide