Steps to reproduce the issue

Install Joomla 3.8.12
Set channel to Custom https://update.joomla.org/core/nightlies/next_major_list.xml
Save&Close
Update

Expected result

3.8.12 updates to 4.0

Actual result

Fail

System information (as much as possible)

Wamp localhost
PHP Built On Windows NT MY-COMPUTER 10.0 build 17134 (Windows 10) AMD64
Database Type mysql
Database Version 5.7.19
Database Collation utf8_general_ci
Database Connection Collation utf8mb4_general_ci
PHP Version 7.1.9
Web Server Apache/2.4.27 (Win64) PHP/7.1.9
WebServer to PHP Interface apache2handler
Joomla! Version Joomla! 3.8.8 Stable [ Amani ] 22-May-2018 14:00 GMT
Joomla! Platform Version Joomla Platform 13.1.0 Stable [ Curiosity ] 24-Apr-2013 00:00 GMT
User Agent Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:62.0) Gecko/20100101 Firefox/62.

Additional comments

According to https://developer.joomla.org/nightly-builds.html

To always use the latest version and test the update component you can use this custom update server: https://update.joomla.org/core/nightlies/next_major_list.xml