I tried the same thing but I couldn't duplicate the problem.. I don't have the Arturia instrument you used so I used a different third-party instrument with one of its factory presets -- after saving & re-opening the Concert, MS did correctly recall the saved preset.
If the only thing not being saved correctly is the CC6 value, then it sounds like MS may be resetting CC values to 0 when you re-load the file. One thing that occurs to me might be to check the Preferences under the General tab. There's a parameter option called "On Patch Change", which [on my system] is set to a default setting Reset to Saved Value, with an alternative setting Keep Current Value -- I wonder if one of those settings might cause a CC value to be changed or reset when you reload the file..? [Btw, that Preference can also be set at the Concert or Patch level, under Attributes].
Of course that might have nothing to do with it -- I'm just guessing -- but it might be worth a look. Or, if the preset has the CC value set to 8 then maybe you need to set it up so MS automatically recalls the preset on reloading or selecting the Instrument, to get it to recall the factory setting for that CC. But again, just spitballing here.. It may be a simple setting, but I'm not a regular MS user, so I don't know the solution off the top of my head..
A search turned up an Apple MainStage Release Notes page that supposedly includes the text "Changes to saved values for screen controls mapped to MIDI CC messages are ... sometimes causes all Smart Controls to reset to a value of 0", but I couldn't find the particular item in a long list of bugfixes and new features on the page -- maybe there's something helpful there, if you can manage to dig it out.. https://support.apple.com/en-us/HT200269
I linked a MainStage course to the thread -- there are videos that talk about Preferences, Preset Setup, Patch Changes, and the like -- maybe there'll be something useful in there..