Lync and automatic reconnect
You would need to turn on debugging in Lync and see what was being logged. As you mention, there is no off switch for reconnect and it does work sometimes with the setting you have - but not all the time.
- https://support.microsoft.com/en-us/kb/2629861
I would wager you need to flush your dns cache per the above link, though, based on experience and your reporting of the issue. Although, there are about a dozen different failure modes contemplated in that guide, so you might start from the top and systematically work through the troubleshooting steps.