This happens fairly often, on Mac and Windows, all versions, and can be discombobulating. When it happens I have to click on another branch, commit, or stash, which will show its info in the other boxes, then click on the one I want, which populates the boxes.
I'm unsure of which condition (or state) the app is in when it happens, so I can't tell you when it's consistent.
Is this happening to you as well? Please say so. This has been going on for the last year and a half we've been using SourceTree, through every version we've had, and we stay up to date.