I made some changes in Control Link - this does update and save the file in the Library/Application Support as per below :
/Users/your_name/Library/Application Support/PreSonus Software/Studio One 4/Surface Data/PreSonus FaderPort (2018).surfacedata
But this is not picked up by the Studio One app which refers to the file in the Devices folder of the package contents of the S1 app. So clearly a Presonus bug - they need to get changes to Surface Data picked up by the app.
So that solves part of the mystery - changes are saved in the wrong place and hence are not retained after restart. Perhaps the easiest thing for people to do is copy the file in the Library path and paste it into the Devices folder in the package contents of the app. You can edit this file manually and this is demonstrated in the forum thread https://forums.presonus.com/viewtopic.php?p=198239#p198239
Over to you Presonus