It’s a strange issue you have. I have some questions for you:
1. Does this only happen on terms? Not posts/pages?
2. Do you have any other field or maybe post/page with the same “slug” (plate)?
3. Did you create these fields directly from within wp-admin? (Not php or json).
4. If you actually check your database wp_options table for this value, does it say “something”?
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