Nested `startTransition` call "takes over" and makes parent `startTransition` unable to track `isPending`.

Seb says it's a bug.

Repro:

https://codesandbox.io/s/pensive-breeze-rg70wn?file=/IndexPage.js:192-288

1. Click the button
2. `isPending` in `IndexPage.js` is `true`

However, `isPending` in `App.js` is `false`.

Expected: `isPending` in `App.js` is also `true`.
if (!hasBeenInitialized) {
            didCheckForLazyInit = true;
            lazyInitGetterStack = getCallerStackFrame();
          } else if (currentlyRenderingFiber !== null && !didWarnAboutRead) {
            if (
              lazyInitGetterStack === null ||
              lazyInitGetterStack !== getCallerStackFrame()
            ) {
              didWarnAboutRead = true;
              console.warn(
                '%s: Unsafe read of a mutable value during render.\n\n' +
                  'Reading from a ref during render is only safe if:\n' +
                  '1. The ref value has not been updated, or\n' +
                  '2. The ref holds a lazily-initialized value that is only set once.\n',
                getComponentNameFromFiber(currentlyRenderingFiber) || 'Unknown',
              );
            }
          }
          return current;
        },
        set current(value: any) {