This post references this bug report reported on 1/31/2011: https://support.ellislab.com/bugs/detail/15145/
Specifically it provides a fix to:
On top of that manually adding a Forum Topic ID does not stick.i.e if I save the entry the field is empty again.
The cause of the problem is in the function function publish_data_db located at approximately line 190 of /system/expressionengine/modules/forum/tab.forum.php.
The function checks to see if the Forum Title and Forum Body exists before trying to update any of the Forum Topic ID. If the Forum Topic ID is not initially set then it can never be set.
The fix:
At approximately line 196 there is the if statement below:
if ((isset($params['mod_data']['forum_title'], $params['mod_data']['forum_body'],
$params['mod_data']['forum_id'])
&& $params['mod_data']['forum_title'] !== '' && $params['mod_data']['forum_body'] !== ''))
{At the end of the if statement located at approximately line 270 add the following elseif statement:
elseif (isset($params['mod_data']['forum_topic_id']) && $params['mod_data']['forum_topic_id'] != '')
{
$topic_id = $params['mod_data']['forum_topic_id'];
$this->EE->db->where('entry_id', (int) $params['entry_id'])
->update('channel_titles', array('forum_topic_id' => (int) $topic_id));
}This checks to see if the Forum Topic ID is set and if it is it will update it.