The most likely cause of this is usually conflicts in field names, either with other fields on the same page or with WP reserved words. It you don’t see anything like this.
If it’s ACF5 can you export a json file of your field group and attach it here. It it’s 4, can you do an export of the field group and save it as a text file and attach that. Will make it much easier for me to test it out.
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