My matrix (multi-configuration) job is configured to
"Run each configuration sequentially"
and NOT "Execute concurrent builds if necessary".

After upgrade Jenkins to 1.609.1 (from 1.580.3),
the subsequent new build will still wait in the queue.
But when the new build starts,
the first configuration build will always be built under "workspace@2"
instead of re-using the original "workspace".

After first configuration build finished,
the subsequent configuration build moves back to use the original "workspace".

This issue is seen only when the build was staying in queue.
If there's no existing build running,
the first configuration build would start correctly under "workspace".