I ran a check on the x-request-id you provided, (for future reference, it’s in the response headers you provided, 2/3rds way down). The JSON contents for this request appear to be for a different product than the one outlined in your original post, but same overall issue. The problem lies with the very first line of your request.
Currently, you’re offering the following input: PUT "/admin/variants/{product_id}.json instead of PUT "/admin/variants/{variant_id}.json
The variants endpoint expects a variant ID to be cited in the call URL rather than the product ID associated with that variant. Because of this, you’ll get a 404 since there are no variants in the shop that match the ID provided. I can’t offer too much information on why this would be happening with these specific products, but I cansay that the issue should be resolved if the variant IDs supplied during the call are updated to match their admin values.
Best,
Graham