EDIT: Ok so I finally got one after 22 boss kills, from the third boss in Skyreach. I was healing on my Holy Paladin and had my specialization set to Retribution so I got the strength Hierloom.
EDIT 2: Five facts about these hierlooms:
I've been averaging 1 per 20 boss kills as have other people I have seen, meaning it has roughly a 5-10% drop rate personal loot from any mythic dungeon boss.
These cannot be won with bonus rolls using Seal of Inevitable Fate.
Similar to the Garrosh Hierlooms, they don't appear in the hierloom tab until you get them
They are 715 item level at level 100, and can be equipped with the Touch of the Void hierloom trinket at the same time, meaning a fresh 100 alt could have both on without worrying about acquiring trinkets.
They drop based on your current loot specialization meaning if you are healing on your Holy Paladin but have loot specialization set to Retribution, the strength one will drop. Afterwards, a random one will drop on that same specializaton, but if you switch to another like holy for example, the healer one will drop.