C4DtoA 22.214.171.124 [ed35daae]
Arnold core 126.96.36.199
New scene. Create a quad light. Change its color to something else. Undo the change (it won't work).
Similar problem occurs if you move the light to some other position in the viewport, change its color, then undo. Both the translation and the color change are undone with one undo action.
Mesh light, quad light, skydome light, and arnold sky are the light sources that seem to have this problem for me.