I've searched all over and haven't found anyone with the same problem, so here goes:
In an area using the Fort Interiour tileset of CEP (latest updated version), the Mythallar 3x3 group tile will not display the four corner tiles in-game, but they do appear in the tileset. In the actual game, they are simply blacked out, and any placeables that cross into those four tiles do not show up at all. However, one of the corner tiles has a pipe that runs from the stone wall to the central platform, and that entire pipe comes up as black.
Here's a picture for reference.Another CEP tileset with a Mythallar 3x3 platform, the Castle Interiour 2 (I believe) will display the tile group perfectly with no issues. This only seems to happen with the four corner tiles of the Fort Interiour's Mythallar tile, and nothing else that I've noticed.
It's not just a display issue on my end, either; when hosting the module for others (all of widely varying specs and graphics cards), none of them can see those corner tiles either.
Is this a known issue, something I can fix on my own, or am I doing something terribly wrong?