I've actually run into this same issue using VMware fusion as well, so its not a problem that's inherent to Parallels.
I think part of the issue is what the VM settings are for you Bootcamp VM. I stopped having this issue once I set the Bootcamp VM to have similar specs to my actual machine, i.e similar processor, etc.
Windows has always had this issue that if the hardware substantially changes, which includes the Processor it does want you to reactivate.