In https://codesandbox.io/s/solitary-smoke-e4ouv (10.1.1):

• The class→className aliasing code (1) seems to mutate the props to add a non-enumerable getter
• the forceUpdate (2) call seems to recycle the mutated props inside props.children (3)
• something weird happens and className becomes enumerable, even though it was never explicitly named (see output)