Bug: SuspenseList revealOrder="backwards" is not consistent without tail props

<!-- 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-aae83a4b9 ## Steps To Reproduce 1. If `<SuspenseList revealOrder="backwards">` is expected to show last component to load first if it loads early but it waits for the top components. If `tail` prop is set it works fine. 2. if `<SuspenseList revealOrder="backwards" tail="collapsed">` is given everything works as expected <!-- 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. --> ``` <SuspenseList revealOrder="backwards"> <ProfileDetails resource={resource} /> <ErrorBoundary fallback={null}> <Suspense fallback={<h2>Loading posts...</h2>}> <ProfileTimeline resource={resource} /> </Suspense> </ErrorBoundary> <Suspense fallback={<h2>Loading fun facts...</h2>}> <ProfileTrivia resource={resource} /> </Suspense> </SuspenseList> ``` works fine when added tail ``` <SuspenseList revealOrder="backwards" tail="hidden"> <ProfileDetails resource={resource} /> <ErrorBoundary fallback={null}> <Suspense fallback={<h2>Loading posts...</h2>}> <ProfileTimeline resource={resource} /> </Suspense> </ErrorBoundary> <Suspense fallback={<h2>Loading fun facts...</h2>}> <ProfileTrivia resource={resource} /> </Suspense> </SuspenseList> ``` Link to code example: [https://codesandbox.io/s/bug-suspenselist-revealordertogether-and-error-boundaries-18429-1oky8](https://codesandbox.io/s/bug-suspenselist-revealordertogether-and-error-boundaries-18429-1oky8) <!-- Please provide a CodeSandbox (https://codesandbox.io/s/new), a link to a repository on …