I have 7 appearances of the legion guardian druid artifact unlocked but only 1 will show up in transmog under artifact appearances and also under fist weapons.
I can right click the image in the transmog window and see the names of all 7 that I have unlocked, but the image does not change as I click alternate names.
I have found a few other people who have verified that they have this same issue as well.
This problem appears to be exclusive to the guardian artifacts. I have all 7 appearances of the other specs’ artifacts available to me as expected.