Do you have any https://www.advancedcustomfields.com/resources/acf-validate_value/ filters on your site. This is where I’d look first for something that could cause this. An example of something that can easily cause this is not not returning $valid from the filter, for example
function my_validation_filter($valid, $value, $field, $input) {
// a statement like this will cause false validation failure
return;
// the above statement must look like this
return $valid;
}
