The Alliance version of the recruit's sturdy stompers from the low level radiant echo drops share an appearance with two items I have collected yet it is not marked as learned despite having the +sources completionist mode turned off.

This is probably due to 2 issues:
The Expeditionary Plate Warboots (alt appearance) are marked as learned with a green checkmark, but also as missing with a red exclamation point in ATT. This used to be true, but they are now rewarded via a quest on Exile's reach (same with the other armor type boots) - (Error in red MISSING IN ATT 108826). As a result, even though they are collected, they are not counting towards the sturdy stomper appearance.

Additionally, there may be a Blizzard coding issue, but the Jingoist's Warboots from Time Rifts also share the same appearance, which I also have collected, but are not shown as an alt appearance on ATT or on Wowhead. The horde boots do show as collected, though and do have this alternate appearance listed, so I am able to use the time rift mog to cross off the horde echoes boots without needing the exile's reach mog counting in ATT. All other armor sets are cross-referenced to the time rift or exile's reach mog and are counting as collected except this one pair of boots.

See the quests under "reward from" to add these boots to "found" in ATT:
https://www.wowhead.com/item=175180/expeditionary-plate-warboots#reward-from-q

Here are the time rift alliance plate boots:
https://www.wowhead.com/item=206871/jingoists-warboots

See how the horde version shares an appearance with time rift (warmonger's warboots) and exile's reach boots, also PVP mogs:
https://www.wowhead.com/item=218035/recruits-sturdy-stompers#same-model-as

but alliance does not share with time rift, only exile's reach boots and other PVP appearances:
https://www.wowhead.com/item=218026/recruits-sturdy-stompers#same-model-as