The statute related to appropriation reductions is KRS 48.600, which would indeed prohibit Bevin from ordering midyear spending cuts in this instance. But Bevin isn't ordering spending cuts in the sense of an appropriation reduction which that statute governs. He's ordering a revision of the allotment schedule, governed by KRS 48.620, which is within his power.
The difference is an appropriation reduction is deemed necessary in statute in the case of a revenue shortfall and an allotment reduction within an appropriation, which this is, can be authorized by the state budget director pursuant to KRS 48.605(1)(a). The legislature can object to the move, but KRS 48.500 clearly gives the Governor the upper hand in such a dispute.