get_results query with accent

I’m trying to do an SQL query and I don’t understand something. I get a value with $ _POST, this value is equal to ‘définition’. I made this request:
$sql = "SELECT DISTINCT * FROM". $ wpdb-> prefix. "posts WHERE post_title LIKE '%". $ _POST ['value']. "% '";.

A var_dump($sql) gives "SELECT DISTINCT * FROM datatablename.posts WHERE post_title LIKE '% definition%'";.

If I do $res = $wpdb->get_results($sql);, I get an empty array

But, if in my code I put directly $sql = "SELECT DISTINCT * FROM datatablename.posts WHERE post_title LIKE '% definition%'"; (I immediately replace $_POST with my value), $res is an array with a post.

The problem stems from the accent, because if $_POST['value'] = 'finition' it’s okay

My data table is in utf8mb4_unicode_ci.

What can be done to solve this problem?