When you enable the statement cache, the QP metrics for ad hoc statements using the statement cache are not instantly dumped into the sysquerymetrics system catalog. Instead, the QP metrics of the respective ad hoc statements are dumped when the statement is flushed out of the statement cache, or by commands such as sp_metrics 'flush' and dbcc purgesqlcache.
You can revert this default behavior by using trace flag T7724.
