Set two terms for a post when they differ only by an accent

Question

I face the following situation: after manually adding (in the backend) the terms with names (not slugs) Mânie and Manie, that differs only in an accent/diacritic (they are totally different words with totally different meanings), to a non-hierarchical custom taxonomy (tags), no matter what slugs were set (I tried the slug pairs minie/manie and manie/manie-2 for that term names), when I try to set these terms to a post, also from backend, when I save the post Wordpress randomly accepts only one of them.

To summarize, the terms are (can be) created / added to the taxonomy, but cannot be added together to the post, one of which being randomly excluded while saving the post.

How can I fix this? I haven't found any useful information so far. The non-hierarchical custom taxonomy was added by a code, but I don't think that's the cause of the problem.

Answer 1

So, WordPress doesn't allow setting two terms (already existing in a taxonomy or being new, no difference) for a post, when editing it in backend (from Edit or Quick Edit) if they differ only by one accent / diacritical mark, randomly rejecting one of them. After several tests, I found that these terms can still be added to the post using a hook/filter - save_post() or save_post_{$post->post_type}. However, there's a problem here too: WordPress will remove randomly one of the two terms the next time the post will be saved/updated if the hook will be disabled, so the only solution I see in this situation is to add the terms in question to the post before every post saving/updating. Given that post savings are not done very often, it is not a big inconvenience.

This is the code (I chose to use the save_post_{$post->post_type} hook):

add_action( 'save_post_your_post_type_slug', function( $post_id ) {
    $taxonomy = 'custom_taxonomy_slug';

    // terms to add; I mention that they were added earlier to the custom
    // taxonomy and the taxonomy may contain other terms for other posts
    $new_terms = array( 'Mânie', 'Manie' );

    // get existing taxonomy terms as an array with their IDs and names
    $tax_terms = get_terms( array( 'taxonomy' => $taxonomy, 
        'fields' => 'id=>name', 'hide_empty' => false ) );

    if( ! is_null( $tax_terms ) && ! empty( $tax_terms ) ) {
        // check if the new terms exist in the custom taxonomy
        $common_terms = array_intersect( $tax_terms, $new_terms );
        if( ! is_null( $common_terms) && ! empty( $common_terms) ) {
            // get IDs of new post terms
            $terms_ids = array_keys( $common_terms );
            // set new post terms
            wp_set_post_terms( $post_id, $terms_ids, $taxonomy );
        }
    }
} );