@fshakhverdiyev Thanks for sending the reproduction project. The problem is that you are using skin bones incorrectly. The to-be-hidden attachment is a normal mesh attachment at a normal bone instead of a skin placeholder:
All bones of the mesh attachment in the weights view are indeed skin-bones which are associated with the respective skin, but this is not how skin-bones work. Bones can be assigned to skins, becoming skin bones, to save performance by automatically disabling the bone calculations when the skin is not active, and make the rig setup a clearer by hiding them in the Spine Editor. You seem to expect it to be the other way round, hiding attachments via skin bones, but it's the attachment (skin placeholder) which hides bones.
Unfortunately it behaves slightly differently in the editor than on the runtime side, hiding the respective attachment as you would like it to look, making your incorrect setup seem to work correctly.
