Not too sure. Most likely, these "cracks" are just crack-like, the material in these areas were melted during welding, but re-solidified when the heat was removed to leave no voids in the material, therefore the fatigue strength will probably not impaired. However, if the melted area is subject to tensile stress, there will be a real crack, which will be buried by the filler, in this case, the fatigue strength will be impaired.
If the parts were heat treated to the T6 temper prior to welding, then the fatigue strength will be lower just due to the alteration of the microstructure. Cracks and possible residual stress changes will further degrade this.
