Updating Drush when installed using pear
Although there is a pear package for Drush-8.x-6.x, if you are going to run this version of Drush I would recommend cloning it from git, and updating it with git pull.  You can also check for newer code with git status instead of drush self-update.

Discussions on the deficiencies of drush self-update should probably happen in the Drush issue queue. Briefly, it no longer handles the update operation itself, and while it works well enough when using stable versions of Drush, it is easily confused in other circumstances (e.g. when Drush-7.x-5.8 is newer by date than the 8.x-6.x-dev that you are running, even though 7.x-5.8 is built exclusively from features backported from 8.x-6.x-dev.)