This applies to the Scarlett 18i20 1st, 2nd & 3rd Gen

The headphone outputs on the 18i20 share their output with whatever is assigned to Line Output 7&8 (for Headphone Output 1) and Line Output 9&10 (for Headphone Output 2).

This means that anything assigned to Line Output 7 in Focusrite Control/MixControl will come out of output 7 on the back of the unit, as well as the left headphone channel for Headphone Output 1. The same thing goes for Line Output 8, with Output 8 on the back and the right headphone channel of Headphone Output 1. The same thing applies for Line Output 9 and 10 with the left and right headphone channels of Headphone Output 2.

In the example below, you can see DAW Playback channels 1&2 have assigned to Line Outputs 7&8 / headphone output 1 and DAW Playback channels 3&4 have been assigned to Line Outputs 9&10 / headphone output 2.