I tried testing this in 5, there is always a post ID and Term ID that match in WP, at least on a fresh install. Though I’m not sure my test is good as it would be extremely difficult for me to get what you have.
First step is to deactivate other plugins and/or switch themes to see if the problem goes away. If that fixes it let us know where the conflict is and we can try to help. If that does not fix the problem then submit a new support ticket https://support.advancedcustomfields.com/new-ticket/
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