Normal. The belt gets loaded when a cam begins on a lobe pair and then after a few degrees another lobe pair is ending and pushes cam ahead. When the belt gets loaded on the segment between crank up to cams the tensioner arm moves right and when the load is relaxed the segment from cams to crank where tensioner resides gets loaded and tighter moving it to the left. The older hydraulic ram tensioner system kept the tension roller from dancing but that meant the belt was taking more shock so they moved to the non or minimally damped eccentric springlike on your engine. That is what the new Drive-E engines use also.