When going through the galley logs, I can see dozens of entries like the following:

2019-07-29T13:14:27.114240Z warn istio.io/istio/galley/pkg/source/kube/dynamic/source.go:147: watch of *unstructured.Unstructured ended with: Underlying Result Channel closed
2019-07-29T13:14:33.114083Z warn istio.io/istio/galley/pkg/source/kube/dynamic/source.go:147: watch of *unstructured.Unstructured ended with: Underlying Result Channel closed
2019-07-29T13:14:34.714333Z warn istio.io/istio/galley/pkg/source/kube/dynamic/source.go:147: watch of *unstructured.Unstructured ended with: Underlying Result Channel closed
2019-07-29T13:15:06.114220Z warn istio.io/istio/galley/pkg/source/kube/dynamic/source.go:147: watch of *unstructured.Unstructured ended with: Underlying Result Channel closed
2019-07-29T13:15:06.514547Z warn istio.io/istio/galley/pkg/source/kube/dynamic/source.go:147: watch of *unstructured.Unstructured ended with: Underlying Result Channel closed

It seems to be caused by the code that watches SMMRs for changes and occurs every few seconds. It takes a few minutes until you see the first log entries (on my cluster ~6min)