Metafields type vs value_type breaks the liquid json function

I followed the directions to migrate to "type" instead of "value_type" for Shopify metafields.
https://shopify.dev/apps/metafields/migration

After switching, I can no longer transform metafields to json in liquid.

This code has always worked:

{{ product.metafields.merch | json }}

Actual output:

Expected output:

Old API code (works):

{
      key: 'preview_image',
      value: JSON.stringify({
        src: previewImage.url,
        contentType: previewImage.contentType,
      }),
      value_type: "json_string",
      namespace: "merch",
}

New API code (breaks liquid):

{
      key: 'preview_image',
      value: JSON.stringify({
        src: previewImage.url,
        contentType: previewImage.contentType,
      }),
      type: "json",
      namespace: "merch",
}

Start by reproducing the Liquid json filter with the issue's old value_type and new type metafield examples. Trace the json filter behavior for Shopify metafields, then add coverage for the new type form and confirm the output matches the expected JSON representation.