hi, in ATT the item https://www.wowhead.com/item=16814 have shared appearance with https://www.wowhead.com/item=10101 and is marked collected in ATT getting the other item, but in the Blizzard side mark the appearance no collected

What about in the Appearances tab and not Sets?

What about in the Appearances tab and not Sets?

show the other item but in xmog NPCs the set is missing again due the pants

I don't think there's any error on ATT part. You have collected other sources of this appearance and they display correctly both in game and in ATT. To collect the set game requires you to collect this specific source of appearance. We don't track sets as a standalone Thing, we track appearances.