get_results query with accent

Your SQL command is highly insecure and open to security issues like SQL injection, so even if this may not answer the question, I strongly suggest you to use $wpdb->prepare() and $wpdb->esc_like() — the latter is used to escape the % character in SQL.

Additionally, you can simply use $wpdb->posts to output the table name for WordPress posts such as wp_posts.

And I noticed that in your SQL command:

The table name is incorrect because the FROM and $wpdb->prefix is concatenated as one word like FROMwp_posts.
There’s a whitespace after the second % in the LIKE clause: %". $_POST['value']. "% ' — so that whitespace is probably not needed? Or that it could be the reason why the query did not return any results.
The var_dump() actually contains no accent — you used definition and not définition. Same goes with the direct one.

Now here’s how your query or SQL command should be generated:

$value = $_POST['value'] ?? '';

// wrapped for brevity
$sql = $wpdb->prepare( "
SELECT DISTINCT *
FROM {$wpdb->posts}
WHERE post_title LIKE %s
", '%' . $wpdb->esc_like( $value ) . '%' );

$res = $wpdb->get_results( $sql );

And I actually tested the above code with a post with the title containing the word définition, and the query returned one result (which is a test post).

If my code doesn’t work for you, you can try sanitize_text_field(), but that will strip HTML tags, among other things.

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