UPDATE - SOLVED
Arturia Support found the solution!
The solution is to create a new Admin account (In Mac OS at least - I don't know if it's the same for PCs).
You don't need to delete your current Admin account, just create an additional new one. Log in with that new Admin account, and your plugins should now show up. Sorcery, I tell you!... My missing plugins did show up, in any case. Curious to know why, but that's for another topic... My thanks to Arturia Support for sticking with me 'till the end!