Error in Validate Field with ACF plugin in WordPress

I did have the same issue. And I wasted enough time for the answer.

At first be sure that:

The ajax request isn’t failed and happens. So, check:

1. Is acf_form_head() before get_header() and run before any html is output?
2. Does your theme contain call to wp_head()?
3. Does your theme contain call to wp_foot()?
4. Are your deferring the loading of JavaScript or otherwise altering the way that JavaScript is loaded on the page?

(Look at this ACF support topic too. If you use acf_form() for creating new user look at this topic).

But in my case the root was is_admin() in this line into the ‘acf/validate_value’ hook:

if ( ! $valid || is_admin() ) { return $valid; }

Because is_admin() returns ‘true’ by AJAX requests. As a result, the validate function didn’t work.

Hope it will helpfully for somebody.