Since the inception of the addition of Ensembles to the game there have been reoccuring issues that have not been addressed or corrected.
Many of the ensembles in the game that have the option of acquiring individual pieces from that set from means, such as PvP. Allow the Blizzard client to break the ensemble by not awarding the full amount of IDs if you purchase it at a later date.
I asked Crieve, the Author(*) of All The Things addon, to explain to us the technical side of how transmog IDs are lost. I will quote him.
“A single appearanceID can have several sources (sourceID) tied to it that grant that appearance. Each Ensemble is created with a set number of appearanceIDs baked into it from specific sources. After a season has ended, a lot of times additional sourceIDs are added to those ensembles. However, if you previously purchased an ensemble, it will report that you’ve collected all of the appearanceIDs from the ensemble despite having those new sourceIDs attached to it. While this is technically correct reporting, since there is no retroactive credit given for having an ensemble learned, those newly added sourceIDs effectively become unobtainable for Completionist players. Attempting to learn the ensemble after the fact will yield the “You’ve already learned all of the appearances” error rather than granting those players their missing sourceIDs.”
For most of the collectors I know. Basically every single one of them. They have arrived at this issue with Ensemble sets from all expansions. Many of us have been playing for over a decade. Allowing the creation of these errors to multiply over Ensembles.
I personally have around 80 missing source(*) IDs which are tracked in the API that I cannot learn by any means. Alts, Same class, phasing tricks. These don’t help.
In the end of it all this is a Quality of Life change that would affect as small percentage of players. It’s also an issue that is that, an issue.