QBS cannot find its plugins in multiarch path

In Debian, we build QBS with the following options passed to qmake:

QBS_LIB_INSTALL_DIR=/usr/lib/${DEB_HOST_MULTIARCH}
QBS_PLUGINS_INSTALL_DIR=/usr/lib/${DEB_HOST_MULTIARCH}

Where ${DEB_HOST_MULTIARCH} expands to a multiarch triplet, i.e. x86_64-linux-gnu.

However, according to a bug report by arch, QBS still tries to search for plugins in non-multiarch location:

WARNING: Plugin path '/usr/lib/qbs/plugins' does not exist.

Is it possible to make it use the actual path? (Otherwise what's the point of making it configurable?)