When not adhering to this order, brightness sysctl's do not show up (tested on Lenovo T490). Thanks to Ulrich for pointing this out! Wouldn't have found it without him.
While I had to do this as well to make it attach, I'm not sure if this is intentional or simply a bug that should be fixed (instead of documented).
Could you try roping in someone knowledgeable in acpi_video or graphics and ACPI in general to have a look? Is this also common for laptops other than Thinkpads for example?