save_on_focus_lost setting is partially broken in build 3183.
If the focus is changed within Sublime Text, switching to another buffer, opening the command palette, etc., then it functions as usual, saving the file. But if you switch to another application then the file remains unsaved.
While checking my installed version using 'About' to write this post I found another problem.
Pressing escape to close the About dialog caused the focus to be auto-changed from one Sublime Text window to another one.
This happened when About was opened from the Command Palette and also when opened from the Help Menu.
However when I repeated the test from the other open windows I found that only one of them resulted in the focus auto-changing to another window. EDIT AMBIGUOUS: It only happened with 1 of the 4 open windows.
When I restarted Sublime Text I was not able to repeat either problem. Both the
save_on_focus_lost setting and the About dialog worked correctly.
I will check again when ST has been running for a while and see what happens...