Based on my understanding you are speciying about double line to ground fault. In double line to ground fault the third phase which is unfaulted should be approximately zero as entire current flows through faulty line. This could be throwing error because of many probable reasons :
1) Firstly checking the design whether you have modelled it correctly or not.
2) Blocks taken for double line to ground fault is correctly selected.
3) There might be some extra resistance which is high in value before fault region because of which most current is flowing through unfaulty line.
There can be n reasons for this happening. Pls look into the designing part more closely.
Hope it helps !