SetLayerToObject is used on an object that resides on a sublayer that is collapsed, the parent layer that is visible in the layer panel doesn’t get painted black (bold typeface).
In the following image, the current layer is set to a sublayer of the “REFERENCE LAYER - TEMPLATE” layer at the bottom of the list.
Often [but not always ], switching to another application and then back to Rhino will paint the list correctly. Just expanding any layer in the list will also trigger correct painting.
While playing around with this for this bug report, I had an instance where a parent layer remained bold when I set another layer as current: