there was never an additional set at t10 in OG, it was 1 at 1-3, 2 at 4-6 and 3 at 7-10. it was first when the shard cap was increased that we got the 4x set and from my memory it only drops at t15
edit: just found the patch notes from when they were added originally
Shard Dungeon Bonus Rewards
You are now eligible to receive bonus rewards when running shard dungeons daily. The bonus rewards are grouped into three tiers.
Tier 1: Shards Level 1-3, 1 reward
Tier 2: Shards Level 4-6, 2 rewards
Tier 3: Shards level 7-10, 3 rewards