So the first thing you need to do is figure out if your filter is actually being called, which is a little complicated since it in a AJAX request.
The first thing you need to to is turn on error logging https://codex.wordpress.org/WP_DEBUG
Then at the top of the function you can output stuff to the error log, for example if you just want to see if the filter is called
error_log('My Filter Was Called');
Then you can look in the error log to see if that appears and that will tell you if the filter is called or not.
