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An assert is triggered in this case because in multi-process PIE, if the editor is a client, the PIEInstance of the world context remains at -1 (see UEditorEngine::PlayInEditor). If this client instance later travels to another map and disconnects from the server, the PIEInstance isn't updated but the WorldType remains PIE, causing the editor to attempt to rename packages, and asserting when the PIEInstance is still -1.
See UDN post
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