I think this issue has existed since Lightfall launched though it could be from last season too. I recently made a post about the Candescent armor set being absent from the transmog/appearence menu but it's been patched, however there seems to be a similar issue with specifically the Deep Explorer Boots on my warlock. For context: I've discovered the entire Deep Explorer suit for warlocks and have transmogged the boots and chest armor. I can see the full armor set in my collections tab and I can see the helmet, gloves, and bond as locked in my transmog screen while the chest is applicable but the boots have disappeared. They don't appear as locked or unlocked, they just don't exist in my transmog screen. Hope this bug report helps!