I attempted to use a Rune of Shattering on an item with a Tier 4 Level of Avalanche prefix and three generic affixes. The Forge panel, when socketed with the item and a Rune of Shattering, showed glowing paths to three of the affixes, including the Level of Avalanche prefix. When I hit the Craft button, I received shards for two of the generic affixes, but did not receive a single Primalist Level of Avalanche shard.
This happened with another +1 to skill affix previously, but I assumed at the time that class-specific shards just could not be created with Rune of Shattering (even though I made sure the glowing paths appeared in the Forge). If this is intended, there is absolutely no indication of it.
Okay. Assuming this is true, the in-game description should probably be changed then, since it says: “The number of Shards created is random for each affix, equal to the tier of the affix at maximum and 1 at minimum.” That is crystal clear that each affix has separate odds with a 1-shard minimum.