I have the same problem and posted another thread on this. At least you got some sort of reply from Parallels - more than I did!
Don't know what it takes to resolve this issue. The pattern is the same, run Windows 10 from Bootcamp, activate with product key, everything OK. Then, boot Windows 10 in VM in Parallels, Windows says 'not activated', go back to Bootcamp, Windows says 'not activated', so then click on 'validate Windows', Windows is now validated, .....
Repeat the process ad nauseum!
This has been going on for months now with many posts on the topic. What does it take for Parallels to find a solution for this crazy situation?