The graph displayed is completely wrong.
I present to 2 views of the graph, one using "All Branches", the other using "Current Branch". Both views are radically different, even though they contain the exact same commits.
The Current Branch view is the correct one, as reported by GitHub, GitKraken. while the All Branches view is wrong.
This is a very serious bug.
SRCTREEWIN-7527 SourceTree Displays Incorrect Git History Graph