Have already tried the following:
1. Use genuine HP zink paper (fresh pack)
2. Charge the printer (is at 100%)
3. Upgrade printer firmware (currently at 1.1.8, last updated 4 Nov 2019, using Android 9)
4. Clean output rollers (used the blue paper from the fresh pack to calibrate at least 10 times)
But the white line is still there.
Wasted so many packs over this and it's really frustrating.