Since I upgraded to 3.3.6, when I launch SourceTree it places a new blank icon on the start bar instead of the Source Tree icon.
I've tried deleting/uninstalling all the old versions of SourceTree, deleting my iconcache.db etc. Having reinstalled SourceTree 3.3.6 I get the following:
There are two source tree exes on my system:
C:\Users\{username}\AppData\Local\SourceTree\SourceTree.exe
C:\Users\{username}\AppData\Local\SourceTree\app-3.3.6\SourceTree.exe
Both of these show as having the normal icon in explorer. If I pin either of these exe's to my start bar then the normal icon appears on start bar.
Having pinned one of these to start bar, if I launch from start bar (or by running exe directly), SourceTree starts up and a new blank white icon appears on my start bar. I can pin this to the start bar

Digging into this, it appears to be because the actual program started is:
C:\Users\{username}\AppData\Local\SourceTree\Update.exe --processStart "SourceTree.exe"
And this shortcut is looking for an icon from a 2.1.2.5 version of SourceTree and the StartIn location is pointing to the wrong place.
I fixed this by editing properties on:
C:\Users\{username}\AppData\Roaming\Microsoft\Internet Explorer\Quick Launch\User Pinned\TaskBar\SourceTree
and fixing the startin folder and changing the icon to point to SourceTree.exe