As I said in a tweet, the fact Sheen was a transfer from NAIT makes his academic eligibility more complicated.
Almost all varsity athletes are considered direct-entry from high school - you rely on your high school grades to get in. If you took some part-time courses while playing hockey, you may get credit from them towards your degree program.
If you were a full-time student at another university, like NAIT, then you are a transfer student. You have to sit out a year (unless you are enrolling in graduate program, or professional degree program like law or education). But going into your "playing year" you have to have successfully completed 18 credit hours in the previous 365 days. (That's why sometimes you see recruits who might have played one term of NCAA or CIS hockey before dropping out and turning pro have to scramble to complete some online courses in the early season before they are allowed to play.)
So it is possible Sheen didn't pass enough credits in his one year at NAIT, or last year if he was enrolled somewhere while playing senior hockey.
That would be my guess, anyways. (We may never know, because student academic records are private, and it is up to the student to disclose them).