There were a couple of issues. The first was that the code assumed the group is not of index 2. This was handled by manually returning the special polygon for this group. The second was that it assumed the special polygon could be obtained from the standard infty, 0, 1 polygon, which is incorrect if the group contains [-1,1,-1,0]. This was fixed by modifying the initial polygon to be infty, -1, 0 in these cases.
There were a couple of issues. The first was that the code assumed the group is not of index 2. This was handled by manually returning the special polygon for this group. The second was that it assumed the special polygon could be obtained from the standard infty, 0, 1 polygon, which is incorrect if the group contains [-1,1,-1,0]. This was fixed by modifying the initial polygon to be infty, -1, 0 in these cases.