Ok .. I've discovered what is stopping copying and pasting envelopes into other tracks.
If 'Paste Special' has been used at all during a session, even not involving envelopes, that will stop copying and pasting envelopes from working. I am able to reproduce this every time.
The solution I have found is to close Sonar completely then reopen Sonar and reload the score. Don't use 'Paste special' at all for anything before working with the envelopes. Using 'Edit Copy' then Edit Paste 'Paste' or 'Ctrl C then 'Ctrl V', I am now able to copy and pase envelopes with ease. Using 'Paste Special' after that stops it from working again.
I'm not sure if this is intended or a bug but it took me hours to painstakingly recreate every envelope and node in a complex classical strings section before I discovered the solution.