Ah, I see. I see that’s what’s happening. I can’t say why the developer first chose slugs for filtering and I’m guessing that it has been maintained this way for backwards compatibility. If it was changed to IDs not then it would break every post object field. Right now all you can do is edit the field and change the filter to the new term name. You can try contacting the developers here https://www.advancedcustomfields.com/contact/
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