The dovecot package puts several helper executables that would normally go into the GNU style "libexec" directory into %{_libdir} (/usr/lib on x86 and /usr/lib64 on x86_64).

This is caused by the line(s)

--libexecdir=%{_libdir}

in the spec file. It prevents the executables from being shared on biarch x86_64 systems between x86 and x86_64 packages. The whole point of having this GNU style "libexec" directory is being able to share helper executables between packages, so it should be something like

--libexecdir=%{_prefix}/lib (no %{_lib})

or maybe even

--libexecdir=%{_sbindir}