To reproduce:

1. Download and open attached project.
2. Select "YoYoYo" folder and notice that in the inspector there is no window for Asset Labels.

Expected result: Folder's asset labels editable and view able just like any other asset.
Actual result: Asset Labels inspector window is gone. Asset Labels can not be seen or edited.

Reproduced with: 2018.1.0a5, 2017.3.0b10, 2017.3.0a2.
Not reproduced with: 2017.3.0a1, 2017.2.0p2, 2017.1.2p3, 5.6.4p3.
Regression introduced in: 2017.3.0a1.

Notes: For reference, select "empty" scene in the assets folder. Asset Labels window is present there and Labels on the scene can be added/edited. Since Labels were original created in 5.6, meta files of these labels still exist. Also AssetDatabase.GetLabels(Selection.activeObject) can still be used with the folder selected where the label was set in 5.6 and the list of labels is returned.

Verified with 2018.1.0b1, 2017.3.0f2.