I have exactly the same issue, and would be grateful for any information.
Although for me it doesn't work even for services published on 10.5, but it works with arcgis services like Demographics/USA_Diversity_Index (MapServer)
Best regards,
Miran
Hello,
There is a bug being logged for JavaScript 4.8 for this issue.
The number is- BUG-000115608 (Cannot the set ON/OFF visibility of the sublayers from LayerList widget in JavaScript application).
You can track the bug via support.esri.com to see the status of the bug.
Harsh