During navigations, the `FlightDataPath` property from the server
response can be an array if there are multiple parallel routes (eg,
`children` and `slot`). When we apply server response to the router
cache, we might call `applyFlightData` for each segment path, which will
copy existing cache values and insert new ones depending on what
changed.
However, the `existingCache` argument that we pass to this function is
the cache at the start of the navigation. That means subsequent calls to
`applyFlightData` will reference the cache _before_ updates are made to
it. This will cause it to erroneously think it needs to lazy fetch for
missing data.
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