The light probe bounding box is not taken into account for object to light probe assignment. The mirror should use the closes light probe from which the mirror is visible. If mirror is not visible from any light probe position, the closest light probe is used.
Perhaps a light probe that you would like to be used by the mirror is not assigned to it, because there is some wall or other object between them blocking the visibility? You can try to move the light probe a bit and see if it fixes the problem.
Could you send us a link to the scene where you see this problem (here or via email to email@example.com) ?