DoT damage is still buggy and will sometimes tick for the non rooted or feared damage, even when visually rooted.
The ghost root happens to me most often when I stand in place after root breaks and I root the mob (key is you can't move after you root it, but can mouse look and spin left/right) but instead of stopping I runs right through me and off into the distance when you turn around. The mob will warp back to his real position when you move but will tick for the reduced DoT damage for a moving target.
Video of mobs ticking for partial DoT damage while rooted
At 30s into the video you can see Chilling Embrace and Asystole ticking for ~4%, then at 50s I step into melee range to take a round of attacks and then get two DoT ticks for 6/7% like it should act if it was rooted. A natural root breaks at 57%, I root it before it attacks me and get 4% moving ticks after it was rooted.