Same problem here...but not just the custom widget. it rearranges everything (back to same order every try).
I tried locking toolbar, same results. tried within a project, both saving and not saving. Also tried with no project loaded.
I also tried by first selecting "all modules visible", and then rearranging. And also with empty bar and inserting one by one and rearranging....same results.
It always goes back to the same original order after restarting Sonar (32-bit).