Happened just today. I was playing Mo & Krill. Heard the Lash ult sound and just quickly clicked my own ult, not realising he was far away and still somehow managed to combo him.
weird indeed. i have blocked seven by using leap. something must have messed up the Z axis.
Because Deathslam ignores the Z axis when calculating the cast range i wonder if there's a weird case where it considers you on the ground or in every point of the Z axis and you found the the one ability that tells us that.