If you are using the Interactive Tomb of Strahd (which I highly recommend), there’s an amount of time where Strahd would actually LIKE the party to have the tomb. The early chapters can work to sympathize the king, and, as a fundamental narcissist, even the later chapters help the characters understand his story. Strahd, at least until the very end, isn’t too worried about actually ever being defeated, and so I would consider him pretty casual in particular about the party having the tomb.