This was improved in 1.2.1 but it still comes up in some scenarios. Please try this fix.
In library/XenForo/BbCode/Formatter/Wysiwyg.php, change:
to:
In library/XenForo/BbCode/Formatter/Wysiwyg.php, change:
Code:
'#<(xf' . $i . ':([a-z0-9-]+))([^>]*)(?<!/)>((?:(?>[^<]*?)|<)*)</\\1>#siU',
Code:
'#<(xf' . $i . ':((?>[a-z0-9-]+?)))([^>]*)(?<!/)>((?:(?>[^<]*?)|<)*)</\\1>#siU',