I’m not really sure about your setup, but I believe that’s how it should be. The relationship should show all options so you can choose any options you want. If you still need to do it, you can try
acf/fields/relationship/query hook to filter the result. This page should give you more idea about it: https://www.advancedcustomfields.com/resources/.
If you don’t know how to code, I suggest you hire a developer to help you out with it, and I’d recommend looking for one on https://studio.envato.com/, https://www.upwork.com/, or https://codeable.io/.
I hope this helps 🙂
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