"Save as" to legacy CS6 , CS5, etc. from CC2018 does not work - although it looks like it does. Trying to open the file in CS6 will get an error that the file was created in a newer version. To confirm the file has not been converted to CS6 (or other legacy version), check the file window. It will display no version number and right-clicking on the file and choosing "get info" will also show no version number.

To work around the bug, launch CS2017 and "save as" to the desired legacy version. You will now see a version listed in the version column of your file window (16 for CS6, 15 for CS5) and choosing "get info" by right-clicking on the file will now show v.16 or v.15.