I updated from SourceTree 2.0.20.1 today straight to 2.1.2.4 and noticed the branches list when viewing a single repo is empty. I'm on Windows 10 x64, system git 2.9.2. Embedded git didn't seem to make a difference.
I double checked that i don't have issues with the git repo itself, command line and such show all my branches as before, so it seems to be strictly a UX issue on SourceTree's part.
Let me know if i can contribute something else.