When a change is made to a person's name it the timeline view, it is not automatically updated in the sidebar index. I came across this while changing the spellings of the last name of each family member whose surname had been entered incorrectly. I chose to use the timeline view to make these changes so that I didn't have to bring up each person's edit screen.
After discovering this, I went into one of the individual's edit screen and confirmed that the changes did take effect. I also tried it with the given name while in the timeline view and had the same result - no change to the index.