This is the single most expensive bug in the whole chain, because everything else about your install works perfectly with it in place — installation succeeds, update detection succeeds, the package's name resolves correctly in Extension Manager. Only one specific thing silently fails: the Download Key field never appears on the Update Sites page, no matter how correct your <dlid> tag is.

Here's the mechanism, straight from Joomla core (libraries/src/Installer/Adapter/PackageAdapter.php):

public function getElement($element = null)
{
    if (!$element) {
        $element = (string) $this->getManifest()->packagename;
        $element = 'pkg_' . InputFilter::getInstance()->clean($element, 'cmd');
    }
    return $element;
}

Joomla always derives your package's #__extensions.element value as pkg_ + your <packagename>. There is no way around this — it's not configurable. So if your manifest declares <packagename>yourpackage</packagename>, your installed extension's element is pkg_yourpackage, full stop.

But when Joomla copies your manifest file into administrator/manifests/packages/, it uses a completely different rule (same file, finaliseInstall()):

$manifest['dest'] = JPATH_MANIFESTS . '/packages/' . basename($this->parent->getPath('manifest'));

That's whatever your manifest file was named inside your zip — nothing to do with <packagename>at all. If you named your manifest file yourpackage.xml (which feels natural, since it matches the packagename), it gets installed as administrator/manifests/packages/yourpackage.xml.

Now here's where it breaks. The Download Key lookup (administrator/components/com_installer/src/Helper/InstallerHelper.php) constructs the path it looks for using the element, not the packagename:

case 'package':
    $path = JPATH_ADMINISTRATOR . '/manifests/packages/' . $element . '.xml';
    break;

It looks for pkg_yourpackage.xml. Your file is named yourpackage.xml. The lookup returns null, getDownloadKey() short-circuits to 'supported' => false before it ever reads your <dlid>tag, and the Update Sites page simply shows nothing — no error, no warning, nothing to grep for in a log.

This is genuinely confusing because two different, unrelated things both get called "package name" in casual conversation, and Joomla itself uses one convention for the filename-on-disk and a different, hardcoded convention for the database element. We only found the exact mechanism by reading three separate core files back to back: PackageAdapter.php, InstallerHelper.php, and the model behind the Update Sites list.