As the title suggests, Pounce appears to check distance from the target's center point rather than its hitbox edge. As a result, against large enemies such as dragons or Tideborn Taleria, Pounce cannot be converted into Carnage even when standing in melee range.
This seems similar to an issue that the Warden's Shalks previously had with Taleria, where distance calculations did not properly account for the target's large hitbox.