While reviewing some code in the forums application I found a couple of places that check the wrong class name for topics if the group is allowed to hide its own topics.
These are the 2 files where IPS\forums\Topics needs to be replaced with IPS\forums\Topic:
Line 649 in \applications\forums\modules\front\forums\forums.php
$canHide = ( \IPS\Member::loggedIn()->group['g_hide_own_posts'] == '1' or \in_array( 'IPS\forums\Topics', explode( ',', \IPS\Member::loggedIn()->group['g_hide_own_posts'] ) ) );
Line 530 in \applications\forums\sources\Topic\Topic.php
$canHide = ( $item ) ? $item->canHide() : ( \IPS\Member::loggedIn()->group['g_hide_own_posts'] == '1' or \in_array( 'IPS\forums\Topics', explode( ',', \IPS\Member::loggedIn()->group['g_hide_own_posts'] ) ) );
I searched all the files just in case, but those are the only ones I found.