When the preparation step by creating face groups has created separate colored areas within a single arcing section, then, if you don’t manually combine the disparate face groups together into one face group, the resulting effort by the prismatic convert will separate those areas, and there will be a line slicing the arc area apart. Carefully checking and combining face groups can help get a better result.
I find myself often in need of an ability to break apart face groups that span across areas where a single face group should not have been that big. I noticed that face groups of small tiny details can get totally missed by the prismatic feature. I think their (Autodesk’s) assumption is that the tool is going to get you part of the way and you need to do some modeling to finish up, but that is very frustrating.