[George]

For anyone that landed here from a Google search. The pagination is not working for forums set up inside BuddyPress groups. You need to copy those two files from
\plugins\bbpress\templates\default\bbpress
pagination-topics.php
pagination-replies.php

and copy them to
\wp-content\themes\[yourchildtheme]\bbpress

Replace the code inside pagination-topics.php to:

<?php

/**
 * Pagination for pages of topics (when viewing a forum)
 *
 * @package bbPress
 * @subpackage Theme
 */

// Exit if accessed directly
defined( 'ABSPATH' ) || exit;

do_action( 'bbp_template_before_pagination_loop' );

// Use bbPress setting for topics per page
$posts_per_page = bbp_get_topics_per_page();  // Dynamically get the value from bbPress settings

// Get the current page
$paged = (get_query_var('paged')) ? absint(get_query_var('paged')) : 1;

// Set up the custom query
$args = array(
    'post_type'      => bbp_get_topic_post_type(),
    'posts_per_page' => $posts_per_page,
    'paged'          => $paged,
    'meta_query'     => array(
        array(
            'key'   => '_bbp_forum_id',
            'value' => bbp_get_forum_id(),
        ),
    ),
);

// Run the custom query
$forum_query = new WP_Query($args);

if ($forum_query->have_posts()) :
?>
    <div class="bbp-pagination">
        <div class="bbp-pagination-count">
            <?php
            // Display the range of topics being viewed
            $start = ($paged - 1) * $posts_per_page + 1;
            $end = min($paged * $posts_per_page, $forum_query->found_posts);
            echo sprintf(__('Viewing %1$s to %2$s (of %3$s topics)'), $start, $end, $forum_query->found_posts);
            ?>
        </div>
        <div class="bbp-pagination-links">
            <?php
            // Generate the pagination links
            echo paginate_links(array(
                'total'   => $forum_query->max_num_pages,
                'current' => $paged,
                'format'  => '?paged=%#%',
                'add_args' => false,
                'prev_text' => __('« Prev'),
                'next_text' => __('Next »'),
            ));
            ?>
        </div>
    </div>
<?php
else :
    echo '<p>No topics found.</p>';
endif;

// Reset post data
wp_reset_postdata();

do_action('bbp_template_after_pagination_loop');
?>

Replace the code inside pagination-replies.php to:

<?php

/**
 * Pagination for pages of replies (when viewing a topic)
 *
 * @package bbPress
 * @subpackage Theme
 */

// Exit if accessed directly
defined( 'ABSPATH' ) || exit;

do_action( 'bbp_template_before_pagination_loop' );

// Use bbPress setting for replies per page
$posts_per_page = bbp_get_replies_per_page();  // Dynamically get the value from bbPress settings

// Get the current page
$paged = (get_query_var('paged')) ? absint(get_query_var('paged')) : 1;

// Set up the custom query
$args = array(
    'post_type'      => bbp_get_reply_post_type(),
    'posts_per_page' => $posts_per_page,
    'paged'          => $paged,
    'meta_query'     => array(
        array(
            'key'   => '_bbp_topic_id',
            'value' => bbp_get_topic_id(),
        ),
    ),
);

// Run the custom query
$replies_query = new WP_Query($args);

if ($replies_query->have_posts()) :
?>
    <div class="bbp-pagination">
        <div class="bbp-pagination-count">
            <?php
            // Display the range of replies being viewed
            $start = ($paged - 1) * $posts_per_page + 1;
            $end = min($paged * $posts_per_page, $replies_query->found_posts);
            echo sprintf(__('Viewing %1$s to %2$s (of %3$s replies)'), $start, $end, $replies_query->found_posts);
            ?>
        </div>
        <div class="bbp-pagination-links">
            <?php
            // Generate the pagination links
            echo paginate_links(array(
                'total'   => $replies_query->max_num_pages,
                'current' => $paged,
                'format'  => '?paged=%#%',
                'add_args' => false,
                'prev_text' => __('« Prev'),
                'next_text' => __('Next »'),
            ));
            ?>
        </div>
    </div>
<?php
else :
    echo '<p>No replies found.</p>';
endif;

// Reset post data
wp_reset_postdata();

do_action('bbp_template_after_pagination_loop');
?>

Then, resave your permalinks.