I had that same problem for a brief moment (lasted around 15 minutes).
OMG, that was the hardest thing for me to figure out as well. I have Paint.NET installed, and it's a breeze in there. In Paint3D it is a major PITA to accomplish simple tasks like cropping.
I noticed that when you tried to open things that were normally opened in Control Panel they opened the Windows 10 Settings page instead. Some of those seek to have reverted to opening in Control Panel again, which makes me wonder if CP is actually sticking around, for this and the above mentioned reasons....