Could you please try to reproduce the issue on one of the WordPress’ stock themes (like Twenty Fifteen) with other plugins deactivated? If it disappears, then you can activate the theme and plugins one by one to see which one causes the issue.
Also, could you please increase the max_vars variable in your php.ini? This page should give you more idea about it: https://www.advancedcustomfields.com/resources/limit-number-fields/.
If the issue persists, could you please provide me with temporary admin credentials so I can check the issue for you? You can share it by replying and setting the post visibility to Private.
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