No problem! Small update on the situation: it only seems to affect certain "build" elements like fences and spandrels.
I have a room that I needed to update due to a furniture cc being glitched. Was able to identify the broken CC via an old gallery upload of that room, which S4TI showed also included two other pieces of CC: a fence and a spandrel. When I uploaded the updated version of the room (without the broken furniture piece), the other two CC pieces weren't showing up on the list. I have since done various re-uploads with other CC pieces (clutter, plants, furniture etc.), and those ARE showing up in S4TI. So it only seems to affect specific build items (wall paint and doors seem to still show up).
2nd Update: I placed all the rooms from this apartment I was working on into a regular residential lot. In particular, I placed the "refreshed" versions where the fences and spandrels don't show up on the CC list of either room (the CC in question is the boundary between two rooms). And even though S4TI wasn't recognizing this CC as being in those rooms, the game still populated the placed rooms with the "missing" fence and spandrel. And can confirm older uploads of these rooms show that CC in the S4TI list for both adjoining rooms.
After this, I uploaded this whole lot to the gallery, and can confirm the fence and spandrel CC DOES show up on the list for the lot upload. So whatever is going on only seems to affect rooms? And specifically anything that is a non-wall boundary between rooms? Kinda specific, but leave it to me to find the one way something can glitch out haha