This is weird - I noticed I'm having this issue with this door as described in this thread:

https://www.leefish.nl/mybb/showthread.php?tid=8743

It also does only display one green square tile so it is indeed like it was only placing one side of the door. I deleted it and rebought it and it still persisted.

The poster there found that a stair wall from MTS was causing the glitch, but I do not have that wall (which is I believe a window).

Just curious whether anyone else has had this or it's a known issue at all, before I do the ol' 50/50.

They did mention the issue wasn't the stair wall fix, as downloading a fresh copy doesn't affect it. Those doors have several CEP extra's, are you sure you have all of them and they are installed correctly?

OK, I've now checked my CEP installation and it seems to be up to date and I have all four components in Documents and Program files. How can I check if I have the extras? I know that I had to download an extra thing to get the Ikea shelf to recolour, so it's possible that I have missed some files.

The extras go in Users\[username]\Documents\EA Games\The Sims 2\zCEP-EXTRA and [Install Location]\EA GAMES\The Sims 2\TSData\Res\Catalog\zCEP-EXTRA. You can compare those files to Numenor's original download, but if you have any custom extras for recolours or anything, you'd need to check the creator sites.