Generate a excerpt from an ACF-wysiwyg-field

You can provide your own implementation of wp_trim_excerpt which is responsible for trimming and stripping HTML tags (it uses wp_strip_all_tags),

By copying the function source code and applying the change that you want (which is keeping the <p> and <strong> tags (or any additional tags as you wish) and it will work nicely.

I copied the function for you and applied the change of allowing <p> and <strong> to be on your final excerpt. (You should put this code in your functions.php file)

function wp_trim_excerpt_modified($text, $content_length = 55, $remove_breaks = false) {
    if ( '' != $text ) {
        $text = strip_shortcodes( $text );
        $text = excerpt_remove_blocks( $text );
        $text = apply_filters( 'the_content', $text );
        $text = str_replace(']]>', ']]>', $text);
        $num_words = $content_length;
        $more = $excerpt_more ? $excerpt_more : null;
        if ( null === $more ) {
            $more = __( '…' );
        }
        $original_text = $text;
        $text = preg_replace( '@<(script|style)[^>]*?>.*?</\\1>@si', '', $text );

        // Here is our modification
        // Allow <p> and <strong>
        $text = strip_tags($text, '<p>,<strong>');

        if ( $remove_breaks )
            $text = preg_replace('/[\r\n\t ]+/', ' ', $text);
        $text = trim( $text );
        if ( strpos( _x( 'words', 'Word count type. Do not translate!' ), 'characters' ) === 0 && preg_match( '/^utf\-?8$/i', get_option( 'blog_charset' ) ) ) {
            $text = trim( preg_replace( "/[\n\r\t ]+/", ' ', $text ), ' ' );
            preg_match_all( '/./u', $text, $words_array );
            $words_array = array_slice( $words_array[0], 0, $num_words + 1 );
            $sep = '';
        } else {
            $words_array = preg_split( "/[\n\r\t ]+/", $text, $num_words + 1, PREG_SPLIT_NO_EMPTY );
            $sep = ' ';
        }
        if ( count( $words_array ) > $num_words ) {
            array_pop( $words_array );
            $text = implode( $sep, $words_array );
            $text = $text . $more;
        } else {
            $text = implode( $sep, $words_array );
        }
    }
    return $text;
}

Notice the line that is doing the stripping:

Line:22 $text = strip_tags($text, '<p>,<strong>');

And your code should be like this:

$project_desc = get_field( 'project_description' );
if( !empty( $project_desc ) ):
    $trimmed_text = wp_trim_excerpt_modified( $project_desc, 15 );
    $last_space = strrpos( $trimmed_text, ' ' );
    $modified_trimmed_text = substr( $trimmed_text, 0, $last_space );
    echo $modified_trimmed_text . '...';
endif; 

For more information, you can checkout this answer on stackoverflow it is more detailed.