I hadn't been using Blender for several years. I forgot a ton of stuff.
I'm doing Subdivision Surface Modifier on an object. I can see it works, when I change the Subdivision resolution or click show/hide in viewport, except for one edge loop. See attached image.
The caps happened to be disconnected from the main cylinder-like body of the object. So I Merged them By Distance. Now it's all one connected mesh. But Subdivision Surface works on all but this Merged edge loop.
Any hints on why this is happening?