Do you see a high loss on 1310 where the 1550 ends. If so you have a badly attenuated fiber. If not, check to see if both index of refractions for both 1310 & 1550 are set properly, but then the difference would show up on all the fibers. Is you cable buried? I have seen similar things happen by the cable getting pinched by rocks and bolders. If you figure it out, let us all know.
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