I have added a variant metafield in the Shopify backend, and called it bis_date_v2. It is currently a string (single_line_text_field).
fetching the metadata for a test variant shows the metafield, with namespace: "my_fields" and the key is "bis_date_v2"
which is what I expect.
When I update the value and then refetch it, the API tells me that the PUT worked (I get a healthy return value, and the refetch shows the new value).
But the value on the Variant's page does not change.