Steps to reproduce the issue

First test

• Install 4.0.0
• go to the options and set this: https://update.joomla.org/core/nightlies/next_major_list.xml as your custom update URL (note#1)

Seccond test

• Install 3.6.4
• go to the options and confirm the default update server is set
• go to com_joomlaupdate and confirm 3.6.5 is shown

Expected result (first test)

Joomla find the update to 4.0.0-dev2 and allow a update to the last nightly version

Actual result (first test)

Joomla don't find any update even not if we fake a 3.6.4 version. (note#2)

Additional comments

In 3.7 everything is fine (using the update server https://update.joomla.org/core/nightlies/next_minor_list.xml)

(note#1)
Please note the download URL in the xml is broken wich is fixed here #14470 and should be deployed with the next run of the nightly build script.

How to fake a 3.6.0 version to the updater (note#2)

• Install 4.0.0
• go to this file: /libraries/cms/version/version.php
• change:
  -- const RELEASE = '4.0'; -> const RELEASE = '3.6';
  -- const DEV_LEVEL = '0-dev';-> const DEV_LEVEL = '4';
• go to this file: administrator/manifests/files/joomla.xml
• change
  -- <version>4.0.0-dev</version> -> <version>3.6.0</version>
• go to the Backend -> Extensions -> manage -> select all -> hit "refresh manifest cache"
• check that the Joomla CMS file Extension has now the version 3.6.4 in the backend.
• set the Default Update server as update source.

Expected result

The Update points to the last version of the default channel (3.6.5) as we fake him to be a 3.6.4

Actual result

No update is there: