The busybox build fails if the CONFIG_STATIC option is selected in the busybox config. The failure is caused by a missing static library libtirpc. The libtirpc package doesn't build a static version by default. The --enable-static option needs to be added to LIBTIRPC_CONF_OPTS in package/libtirpc/libtirpc.mk
This is not a bug. By default Buildroot has the option BR2_SHARED_LIBS=y, which means "build only shared libraries, link everything dynamically". If you set BR2_STATIC_LIBS=y instead, then only static libraries will be built, and everything will be statically linked. If you set BR2_SHARED_STATIC_LIBS=y, then both static and shared libraries will be built (when possible: some sub-par build systems only allow building either static or shared libs, not both), and programs will generally be linked dynamically, unless their build system explicitly links statically.