This is not a major issue, but it is kind of annoying. The first time trials was out for this season (S22), I, along with my clan mates decided to grind out trials for the armor because we thought it looked cool. After some time, we got the full set and wanted to transmog the armor, however, we found a bug where the hunter chest piece just doesn't show up. All armor pieces for the other classes and the other armor pieces for hunter allow you to transmog them; just not the hunter "Photonic Vest."
Like I said, it's not that big of a deal, it would just be nice to have the full set to continue the drip game.