Or at least that's what I think is happening.
I recently enabled IPv6/dual-stack on my server and Mumble now shows both the IPv4 and IPv6 address in the server browser. My client system however has no global IPv6 address and only uses IPv4 to communicate with the outside world. It looks like Mumble is trying to contact the server over both IPv4 and IPv6, which results in a loss of exactly half of all packages sent (see the attached screenshot).
This only happens in the server browser, there is no packet loss after connecting.
I am on Debian Sid and my Mumble version is 1.2.3-348-g317f5a0-1. I also tested this on a Windows 7 PC. The server browser does not even show the IPv6 address and there is no packet loss.