Can't be. If you lost a year of eligibility for greyshirting, no one would ever greyshirt. There would just be no advantage to it. And greyshirts always enroll part-time and there must be a reason for this.
Now, I could believe that the eligibility clock starts ticking after you enroll full time for the first time. In other words, part-time enrollment only protects eligibility if you have never enrolled in school fulltime in the past.