If a package is Architecture: all only (or I guess has some non-all binaries
that don't build for the current arch), then building the package produces a
_arch.changes, rather than an _all.changes.

This is slightly underdocumented in the manual page, it says:
"for  a  build  that  includes  any  the  name  will  be  source-name_source-version_arch.changes"

without mentioning that build actually means --build.

It might be a better idea to produce _all.changes files if no non-all binaries
are produced though (and _all buildinfo files).