Steps to reproduce the issue

Created some tags (with varying length and some with more than 1 word)
Tried to add the tags with the auto-fill-feature (only typing a few characters and then selecting the suggested Tag)

Expected result

The text should reset and another tag can be added, starting with an empty string.
Also the cursor should remain in the input-box.

Actual result

Sometimes (it most often occurred when i had to retype because of a typo [Meaning i first typed "fino"->"fin"->"fini"]) the string didn't reset after i filled in the tag.
E.g. I typed "fini" and then selected "finished Projects", but then i had the tag added and after that the
"fini" was still there. Plus somehow my cursor left the box, meaning pressing backspace didn't remove the string,
but instead triggered the browser's "go to last page".
Besides, sometimes the problem would solve itself when tabbing out of the window and in again.

System information (as much as possible)

PHP Built On Linux CanisMajor 3.10.35 #5644 SMP Wed Oct 28 12:37:28 CST 2015 x86_64
Database Version 5.5.44-MariaDB
Database Collation utf8_general_ci
PHP Version 5.5.30
Web Server Apache
WebServer to PHP Interface fpm-fcgi
Joomla! Version Joomla! 3.4.5 Stable [ Ember ] 22-October-2015 21:30 GMT
Joomla! Platform Version Joomla Platform 13.1.0 Stable [ Curiosity ] 24-Apr-2013 00:00 GMT
User Agent Mozilla/5.0 (Windows NT 6.1; rv:42.0) Gecko/20100101 Firefox/42.0

Additional comments

Note: This was produced on a modified Version of Joomla maintained by Synology Inc!
I will check if this bug appears on my other platforms, too.