The code you’re using will cause acf to save all of the fields to whatever post has the id of ‘300’. I’m not sure you’re using this correctly, to be honest I’m not sure where they values are being saved because you have 300 as a string and on a number. The values could be saved to the post with the ID of 300 or they could be saved in the options table with a prefix of “300-“. If they are saved to a post then there is no searching required. If they are save to the options table then searching is not possible.
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