Hi. As noted in the reference, global.title_tag and global.description_tag should be included in seoInput, not in metafield, to resolve the issue.
{
"productSet": {
"id": "gid://shopify/Product/7851971248188",
"seo": {
"title": "The Demo product",
"description": "The product is a test product"
},
"metafields": [
{
"key": "brand",
"namespace": "custome",
"type": "single_line_text_field",
"value": "My Brand"
}
]
},
"synchronous": true
}