There are a few items I have received on alts that are not showing up in the collection logs for the respective toons, IE demon hunter demonbane leggings are sitting in my hunters inventory after receiving the drop but they aren’t credited as collected.
Those pants are restricted to Demon Hunters, and at this point the developers haven’t yet enabled the transmog to be learned by anyone other than Demon Hunters. All the tier gear from Legion and Tiers 1 and 2 from vanilla are like that: they drop the actual gear, it’s class-specific and soulbound, and therefore cannot yet be learned by anyone who didn’t loot it.
The devs will change that sometime after TWW launches, but they haven’t gotten to it yet.
Have patience. If you can spare the storage space, hang on to those and they’ll become retroactively available for learning transmogs.
Otherwise, expect not to learn the class-specific tier items from Tiers 1, 2, 19, 20, and 21.
It did work with multiple other pieces of the sets though which is weird. But thanks for showing me that post I appreciate it!
Not every tier piece is class-restricted. It’s mostly the “core” tier pieces which used to give the set bonuses. The non-class-restricted armor bits were the focus of the last transmog learning change.