I don’t know when it started happening, I only noticed in early 4.4, during an effort to clean up warnings. I hoped that it will go away with a new version when I migrate everything to an empty project, but it didn’t.
A few more clues: The warning also shows up every time the parent class is compiled, compiling the child yields no warning. Deleting the set transform nodes doesn’t actually help, I have to remove the scene component from the child BP. Conversely if I add any new component to the child (scene, light, etc), I get a new warning for each one. Any new child with additional components will produce extra warnings as well.