"UO" outliner detection flag in istio_requests_total metric is missing ,but see a flag with "-"

Question

I have a weird issue when trying to check circuit breaker metric . The query I used is

sum(istio_requests_total{response_code="503",response_flags="-",reporter="destination"}) by (pod_name,source_app, response_flags, response_code,destination_service)

The result shows me the following output

{destination_service="XXXXXX.namespace.svc.cluster.local", pod_name="XXXXXX-5ff5f6bcc5-xv7gc", response_code="503", response_flags="-", source_app="serviceC"}
107
{destination_service="XXXXXX.namespace.svc.cluster.local", pod_name="XXXXXX-5ff5f6bcc5-xv7gc", response_code="503", response_flags="-", source_app="serviceA"}
67
{destination_service="XXXXXX.namespace.svc.cluster.local", pod_name="XXXXXX-5ff5f6bcc5-rzgd9", response_code="503", response_flags="-", source_app="serviceC"}
105
{destination_service="XXXXXX.namespace.svc.cluster.local", pod_name="XXXXXX-5ff5f6bcc5-rzgd9", response_code="503", response_flags="-", source_app="serviceA"}
75
{destination_service="XXXXXX.namespace.svc.cluster.local", pod_name="XXXXXX-5ff5f6bcc5-xkwff", response_code="503", response_flags="-", source_app="serviceC"}
113
{destination_service="XXXXXX.namespace.svc.cluster.local", pod_name="XXXXXX-5ff5f6bcc5-xkwff", response_code="503", response_flags="-", source_app="serviceA"}
64
{destination_service="XXXXXX.namespace.svc.cluster.local", pod_name="XXXXXX-5ff5f6bcc5-gdptg", response_code="503", response_flags="-", source_app="serviceC"}
103
{destination_service="XXXXXX.namespace.svc.cluster.local", pod_name="XXXXXX-5ff5f6bcc5-gdptg", response_code="503", response_flags="-", source_app="serviceA"}
73
{destination_service="XXXXXX.namespace.svc.cluster.local", pod_name="XXXXXX-5ff5f6bcc5-kpnzq", response_code="503", response_flags="-", source_app="serviceC"}
109
{destination_service="XXXXXX.namespace.svc.cluster.local", pod_name="XXXXXX-5ff5f6bcc5-kpnzq", response_code="503", response_flags="-", source_app="serviceA"}
69
{destination_service="XXXXXX.namespace.svc.cluster.local", pod_name="XXXXXX-5ff5f6bcc5-d7msw", response_code="503", response_flags="-", source_app="serviceC"}
107
{destination_service="XXXXXX.namespace.svc.cluster.local", pod_name="XXXXXX-5ff5f6bcc5-d7msw", response_code="503", response_flags="-", source_app="serviceA"}
72

Only UO is missing (pertaining to circuit breaker ) I can see following flags for 503 -> UC (UPSTREAM_CONNECTION_TERMINATION) ,UF (UPSTREAM_CONNECTION_FAILURE)

is this some issue with response flag ? please help