Unfortunately I cannot reproduce the issue you are describing. When I modify the character of the Spine Examples/Other Examples/Sprite Shaders scene, it works both when I flip the character via negative x-scale or via 180 degree rotation. However if I disable Solve Tangents then the character receives no light at all. Do have this setting enabled?
If this does not help, I suspect some negative scaling in the hierarchy above maybe? You could try moving the Spine skeleton all the way up the hierarchy to be one of the root nodes, and see if the problem persists. If yes, could you please post the inspector settings of your character? Or even better - send us a minimal project that still shows the problem and send it to contact@esotericsoftware.com? That would help us a lot in resolving your problem!
