Poxbursters are immune to push stagger when they are on a different elevation from the player. This includes the player being at the top of a stairway (the most easy way to reproduce), while the Poxburster is on it and even just particularly rough terrain with protrusions sticking out of it (which may constitute a change in elevation). This only effects melee weapon push function and the Psyker’s “Psykinetic’s Wrath” ability, not melee or ranged attacks.
Steps to Reproduce:
Stand on different elevation from the Poxburster as it attempts to detonate and attempt to push it.