Add the following to the top of your function to test
die('My filter is running');
try to save the page and see if it appears. Remove the code as soon as it does.
Possible corrective actions
1) Set the priority for your action > 10
2) Make sure you followed all of the steps for using front end forms https://www.advancedcustomfields.com/resources/acf_form/
— did you include acf_form_head() before get_header()
check your theme, make sure it includes both wp_head() and wp_footer() calls
