This is a known issue that has been reported with 4.9 with the introduction of Static Lights being captured in the Reflection Captures.
It was reported with UE-19818. Ideally you would be able to use the min-roughness setting to reduce light acne that is caused by this, but that’s a limitation of the implementation at the moment.
You can try increasing the source radius of the static light to see if that helps reduce the artifact though.
I hope that helps.
Tim