Any changes to meta data from code will not work since Whatsapp will never run your scripts. So whatever is defined in the static html will be what it takes. And Anvil just sets the og:image metadata to your app logo by default.
But you will need to make your need clear. It could be either -
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You want different images for different pages. In this case, I had made a feature request for it - Support for Image in routing meta
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You want your WhatsApp preview to show something other than just your app logo (favicon). In this case, it is not a routing issue.