I just happened to be in the dev kit when I saw this one. So I was able to reproduce this error in the current version of the game too, though I may have spotted something you didn’t (or at least is apparent in live but not testlive).
Here we have our lovely building, 2 pillars with a wedge.
Now I want to upgrade the wedge
The upgrade is complete, however it didn’t replace the wedge, the two pieces merged together instead. I used black ice as an example to try and make it more visible, but I tested it with other tiers and versions as well.
When the wedge isn’t in-between two pillars, it works like normal when upgrading. A normal square ceiling that is in-between two pillars also upgrades correctly.