Hi,
I just logged on after the maintenance finished and my characters that previously had shirts transmogged to something else had their appearances reset back to stock and now when I try to mog them back it just says “thisitem cannot be transmogrified”.
This happened to me, too. The white tuxedo shirt I have on used to be transmogged as the lumberjack shirt and now it’s not. I also can’t re-transmog it. I have not checked all of my characters but this one was obvious since the chestpiece is hidden.
Have the same issue with regards to the White Tuxedo Shirt. Logged on to my druid alt from the character select screen, and imagine my surprise when her shirt slot is no longer hidden.
UI also refuses to let me change it back to hidden or anything else.
I wonder if this has something to do with it. Like the cosmetic identifier overrode some transmog ability
https://www.wowhead.com/news=319209/boe-cosmetics-now-have-pink-corners-in-your-bags-undocumented-9-0-2-change
I noticed that myself on Tuesday when the maintenance was concluded. Don’t recall the White Tuxedo Shirt being flagged cosmetic prior to patch 9.0.2, either, but either way Blizzard definitely broke something.
After doing some further digging, it also looks like the White Tuxedo Shirt has disappeared from the Appearances tab entirely. 
I checked again once the patch 9.0.5 PTR was up, but unfortunately the issue(s) here with regards to shirt transmogrifying have still not been fixed.
Edit: Patch has since gone live and still no fix(es).
With the PTR for Patch 9.1.0 now being available, I checked yet again to see if anything had been corrected. And, yet again, the answer is still no.
11/17/2022
Forced to update here because of dumb forum rule.
My last post in this thread.
As of patch 10.0.2, the issue has still not been corrected. I can only assume at this point that the change was intentional and no one here should expect a “fix”.
