I opened an existing Illustrator .ai file. Changed the colour of an object's stroke. Saved. Closed. Then I noticed the change was not there.
Repeated several times. Kept trying to make the change and command-s. 
I could see the colour change when I changed the value, but saving did nothing. I noticed that after making the change, the red circle at the top left of the window did not get a black dot to indicate an unsaved change.
I finally did a Save As to make it remember the change. Using Illustrator 24.2 on Mac OS 10.13.6