Although m2 looks rather isolated, it can indeed be covered by wyz (with
m6) or by xyz (with m10). After choosing the essential prime implicants, the
remaining three 1’s can each be covered by two different prime implicants.
Since three 1’s still need to be covered (after choosing the essential prime
implicants), and since all the remaining prime implicants are groups of two
and thus have three literals, we need at least two more of these prime
implicants. Indeed, there are three ways to cover the remaining 1’s with two
more prime implicants. Using the first criteria, we choose one of the prime
implicants that covers two new 1’s, wyz, as shown on the left-hand map.