Imagined Burden in Domination Specialization disables the knockback of GS Illusionary Wave skill. Even though the tooltip still says “Knockback: 450” with Imagined Burden active, when Illusionary Wave is used the NPC foe gets interrupted but most of the time (roughly 90%) there is no knockback. Haven’t tried it out in PvP or WvW.
When I change the trait to something other than Imagined Burden, the predictable knockback of Illusionary Wave returns to normal.