I also ran into a similar problem where the ajax validation had mysteriously stopped working, then come back, then stopped working again. The if( !acf_verify_ajax() ) die(); tip above helped me narrow it down to a wp_verify_nonce() fail inside that function, which apparently was due to the admin being forced over https when the front end form was still over http. I’m not sure this would solve the original poster’s issue, but for anyone who had the ajax validation mysteriously disappear on them, I would suggest trying to load the form over https if the admin is forced over https.
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