this was always a bug, as shadwdrgn really wanted people to grind for their shards. but because it was so much work, he added the soul stealing enchant for people, but he didn't fixed the anvil bug fast enough and FTB started to share the mod in their modpacks and people were told by others it was a "feature"
right now Soul Shards 2.0 got released for 1.6.4 with some new changes, I haven't tried it out though as I kind of prefer MFR's kind of spawning then Soul Shards kind of spawning...