I found the issue. It seems that Hellhound Syri never has a "default" outfit in her bust list, so that unknown image comes up if you swap to the greatcoat and try to talk to her--the greatcoat only ever appears if the scene calls for it directly it seems.
Hopefully fixed for the next release. Because she only ever swaps between the Steelesuit and greatcoat, if Hellhound Syri is not nude or maid, if she's only wearing the greatcoat, she should now correctly use her greatcoat bust art. For any scene where she is forced to wear her greatcoat on top, regardless of what she is wearing underneath, those busts should appear like usual.