I run
{{{ rpmdev-bumpspec -n 1.29-2 rhpkg.spec }}}
and Release is changed
{{{#!diff -Release: 1%{?dist} +Release: 2 }}}
%{?dist} is lost. Is this a bug, or by design?
I'm using rpmdevtools-8.9-1.fc23.noarch
By design, see the man page. If you want %{?dist} there, pass it, i.e. {{{rpmdev-bumpspec -n 1.29-2%{?dist} rhpkg.spec}}}. In most cases I suppose you wouldn't need to use the -n option in the first place though, especially for release only bumps.