The feed controller calls \parse_attachments, which will try to use the MISSING_INLINE_ATTACHMENT language string if an invalid [attachment] tag is encountered, but this string exists only in the viewtopic.php dictionary and this dictionary is not loaded by the feed controller (or more accurately, it’s not loaded by \parse_attachments, which is the component which requires this key), so it triggers a notice.

Backtrace (3.2.9):

PHP Notice: Undefined index: MISSING_INLINE_ATTACHMENT in includes/functions_content.php on line 1369
#0 phpbb/feed/helper.php(158): parse_attachments('25', '...', Array, Array)
#1 phpbb/feed/controller/feed.php(351): phpbb\feed\helper->generate_content('...', '20ae17y2', 'gAg=', 7, '25', Array)
#2 phpbb/feed/controller/feed.php(273): phpbb\feed\controller\feed->send_feed_do(Object(phpbb\feed\topic))
#3 phpbb/feed/controller/feed.php(242): phpbb\feed\controller\feed->send_feed(Object(phpbb\feed\topic))
#4 vendor/symfony/http-kernel/HttpKernel.php(135): phpbb\feed\controller\feed->topic('27940')
#5 vendor/symfony/http-kernel/HttpKernel.php(57): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(phpbb\symfony_request), 1)
#6 app.php(35): Symfony\Component\HttpKernel\HttpKernel->handle(Object(phpbb\symfony_request))
#7 {main}