Bug: Nested SuspenseList may display fallbacks while the component is loaded on re-render

<!-- Please provide a clear and concise description of what the bug is. Include screenshots if needed. Please test using the latest version of the relevant React packages to make sure your issue has not already been fixed. --> **React version:** 0.0.0-experimental-e5d06e34b Hoping this can be helpful for you. Here is what looks to be a bug with concurrent mode and nested SuspenseList. ## Steps To Reproduce _In concurrent mode only_ 0. Setup Let's suppose we have three components `<A />` (**not** lazy loaded), `<B />` (**not** lazy loaded) and `<C />` (lazy loaded). In other words: ```jsx import A from './A'; import B from './B'; const C = React.lazy(() => import('./C')); ``` 1. Render ```jsx render( <SuspenseList key="1" revealOrder="forwards"> <SuspenseList key="1.1" revealOrder="forwards"> <Suspense key="1.1.a" fallback={<div>Loading A</div>}> <A /> </Suspense> </SuspenseList> </SuspenseList> ) ``` 2. Update the component (component now shows A, B, and C) ```jsx render( <SuspenseList key="1" revealOrder="forwards"> <SuspenseList key="1.1" revealOrder="forwards"> <Suspense key="1.1.a" fallback={<div>Loading A</div>}> <A /> </Suspense> <Suspense key="1.1.b" fallback={<div>Loading B</div>}> <B /> </Suspense> <Suspense key="1.1.c" fallback={<div>Loading C</div>}> <C /> </Suspense> </SuspenseList> </SuspenseList> ) ``` 3. Output is: `A / Loading B / Loading C`. While `B` has already been loaded (not lazy loaded). If I understand well the behaviour of `forwards` I would have expect to have `A / B / Loading C` instead. Please note that the behaviour is not the same if I do not use nested `<SuspenseList />`. <!-- Your bug will get fixed much faster if we can run your code and it doesn't have dependencies other than React. Issues without reproduction steps or code examples may be immediately closed as not actionable. --> CodeSandbox: https://codesandbox.io/s/…