This may happen because supply side policies of reducing the marginal tax rates will increase the aggregate supply as well as the aggregate demand simultaneously. Aggregate supply will increase because workers will supply more labour hours and produce more units. aggregate demand will increase because taxes are reduced and disposable income is increased which increases consumption

Because rightward shift of the aggregate supply curve will reduce the price level and increase the GDP, and rightward shift of the aggregate demand curve will increase the price level and increase the GDP, we can see that GDP is increased from both sides but there is no change in the price level, assuming that the size of the shift is same. due to this reason it is expected that there will be no inflation from supply side policies and only GDP will be pushed up.