Ok, I've looked into this more and things are definitely a mess.
Using the latest version of Jup-8 (installed today):
If I set up midi mappings in Nuendo and use 'Save Current Config' and the file name 'XYZ', the file is automatically saved to C:\Program File\Arturia\Jup-8 V3\ as XYZ.jupmidi. I notice that, in addition to the cc values, all the mappings also store the midi channel that the plugin was set to respond to in Nuendo (example 'cc103, Ch 1'). In this case the midi channel used was '1'.
If I use 'Export Current Config' option and select the same folder as the default one (as above) and use the exact same file name, it is saved to the same location. (I also tried using a different file name).
Now, if I then use the Standalone version, regardless of whether the .jupmidi file was saved using 'Save Current Config' or 'Export Current Config' the midi mappings are not responded to. If I then look at the mappings in the Standalone version by selecting the Midi icon, all of the mappings that were set up with the plugin version ARE there (example 'cc103, Ch 1') but are ignored. This is because, for some reason, the Standalone version expects the mappings to be set to Midi channel '2' - God knows why. I can find no place in the Standalone version setup to choose a midi channel. Should it not ignore any midi channels?
If I then redo the mappings using the Standalone version, it will always use midi channel '2' by default. So then back in Nuendo, the plugin does not respond to the midi mappings made in the Standalone version - even if the plugin's midi channel is set to '2' or even 'ALL'!
So, regardless of whether 'Save' or 'Export' is used, the system does not work.
As I said - another fine mess. Unless I'm missing something.