Can you try changing the name of the class and recompiling your C++ project? Make sure it’s something unique just to test it out. Sometimes if you use a class name that already exists in the codebase, UE can do very strange things. It’s happened to me several times. If that’s what it is, then you can try and pick a new name that still suits your project.
Also, make sure that you did a build in Visual Studio BEFORE parenting. Don’t parent a class that was added and compiled while the editor is open. That will only last for the session because live compile substitutes class names until you do a rebuild in Visual Studio. This is an especially common problem with new variables in C++.