Hi Rick,
This problem cannot be reproduced. Preferences settings are always saved on all supported platforms. In the sequence of steps you have posted, step 9 always reflects the changes to preferences made in step 3. This happens all the time, on all of our working and testing machines, on all platforms.
This is a machine-specific issue that has not been reported by other users. Check if you have some virus protection or file management utility running on your machine, which could be blocking changes made to PixInsight's configuration file. As Rob has pointed out, the file is "~/Library/PixInsight/PixInsight.plist" on macOS.