I have had a similar experience with this tippet material.. I bought two spools of it at Pickering Store, made up some leaders using a double nail knot, which i really like, and when i pulled to test the knot , they snapped right at the knot.
I tried several times, and used a couple of different knots, but same results.
I took it back to the store i got it from, and the fishing chap there tried it using his style of knot, and it also snapped for him--several times.
Not experienced this with Orvis line before this, but it was definately weak. The store was excellent as usual and refunded it to me. The bloke there said he would prefer to refund it, rather than me take another spool and get the same results.