No. PoweRec fail-safes to ON. It is active as long as it isn't explicitly disabled by software. The fail-safe check is done every time you insert a new disk.
So if you want PoweRec disabled while burning, you'll need to do so explicitly in your burning software, or, if that software does not offer this, use Plextools to do so and keep Plextools running in the background while you are using that burning software.
Most likely you are observing a slight variance in media quality that is normal at high speeds like 16x.