amd64 vs. x86:64 architecture mismatch in FreeBSD-base repo

[Mel Pilgrim]

I'm trying out pkgbase, using the BETA2 release with src checked out at 
r303326 (current at the time of checkout).  I've run the buildworld, 
buildkernel, and packages targets, and added the appropriate bits to 
/usr/local/etc/pkg/repos/FreeBSD-base.conf, but when I try to get the 
catalogue, I ran into a problem with ABI mismatches:

# env ABI="FreeBSD:12:amd64" pkg update -r FreeBSD-base
pkg: Warning: Major OS version upgrade detected.  Running "pkg-static 
install -f pkg" recommended
Updating FreeBSD-base repository catalogue...
Fetching meta.txz: 100%    264 B   0.3kB/s    00:01
Fetching packagesite.txz: 100%   48 KiB  49.0kB/s    00:01
Processing entries:   0%
pkg: wrong architecture: freebsd:12:x86:64 instead of FreeBSD:12:amd64
pkg: repository FreeBSD-base contains packages with wrong ABI: 
freebsd:12:x86:64
Processing entries: 100%
Unable to update repository FreeBSD-base

I fixed that problem by renaming the directory and repointing the 
"latest" symlink (why wasn't it relative?), but this seems like a bug 
where one part of the repo building code is using the running system's 
architecture, and another part is using the src tree's architecture. 
Why doesn't pkg treat these two labels as equivalent?