Made a group of 6 X 6 platforms for religious altars, and only the Jhebbal Sag Altar will not place. Have tried rebuilding from flotsam and DLC material, and placing on foundations . It still won’t place on any building material.
The Jhebbal Sag altar has always been a little tricky to place. I think it actually has a larger footprint than the others. When you upgrade it, it takes up a lot more room in the front and back than the first tier version.
I also placed one yesterday on a bed of foundation blocks, so I’m fairly sure that’s your problem. You need a bigger platform.