I found this thread due to also encountering this issue in UE5.3.0.
Create a new blueprint from a class I created, or change the parent class of an existing blueprint. Save everything, close everything down and come back to this.
I also noticed that the C++ classes folder would not show up until I started Visual Studio from the Unreal Editor.
Curiously to me, a ton of classes were not listed when trying to set a new parent (from non). Choosing one of the still possible parents, then trying to undo results in a fatal error.
Solution #1: is to delete the binaries folder in your project.
Solution #2: is to open only your Visual Studio solution and build your project.
Hopefully someone discovers these solutions and it helps them too 