Wake turbulence does not always decrease with altitude. We used to fly three of four aircraft in loose formation out to an island weekly. Often one aircraft (GA piston types) would be up to 5 miles behind the other due to ATC delays and sometimes would experience wake turbulence from the preceeding aircraft in cruise at the same level. One day we allowed one aircraft to catch up to the other (both on AP) to see the altitude difference - there was none.