What field type is ‘genrelist’?
If it is a post_object or relationship field, you can’t save “MYSTERY” as the value, you instead need to save a numeric value of the post’s ID.
If the field type is a text based field, your code should work.
If the field ‘genrelist’ is a new field and has not yet been saved on the fields, please read over the docs below to understand why you need to use the field_key instead of the field name
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