Some of the empty-set throws were pre-snap reads, suggesting Burrow had decided where to throw the ball before he'd even taken the snap. In a situation like that, Boyd or anyone could theoretically streak wide open and it's irrelevant, because Burrow has already elected to throw to Mixon (i.e. into the back of the defender blanketing Mixon). The same thing happened at least twice with a blanketed Higgins.
Is that Burrow's fault? Probably to some extent. I can't imagine that's happening though without it being a deliberate element of the game plan and the play call.