At all other tiers they only give RAW materials though. That's why I think it's a bug.
Correct. I've been seeing one unit of refined material on occasion, in addition to the raw mats, ever since my first character reached top tier in crafting. At all other tiers (I still have some younglings who are at lower tiers), I've only ever received raw mats.
If it is a bug, it's been in the game for years - ever since tier 10 mats were introduced to the game.