That shouldn’t happen unless the map the engine loads at the start has all the assets in it. Which is usually not the case. Don’t know why that’s happening.
But another thing you could try instead of migrating :
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Copy the Config folder
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Add the assets to your project.
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Paste the copied Config Folder back and replace the files. (preferably make a backup of the files before replacing them.)
This should keeps the project settings the same as before adding the assets.