Buff Immunity & on hit buffs on a target.

I would have thought that when a toon inflicts buff immunity then this would be applied to the target before processing any on hit buffs.

If we look at b2's special 'mow down' it says:
deal physical damage & dispell all positive effects on them with a 65% chance to inflict buff immunity for 2 turns (applied before damage).

So riddle me this why does using b2's special on sith under a zMaul lead result in sith being stealthed and with buff immunity? So from my reading of the ability the following should happen in order

1 attack resolves as hit or evade - on evade go stealth
2 on hit apply dispell and resolve buff immunity
3 on hit resolve critical hit and apply physical damage

So if it misses then sith should go stealth and gain tm - all good. But if it didnt hit no buff immunity.

If it hits it should dispell, resolve whether buff immunity will proc. So at this point we should see dispell all buffs and buff immunity on 65% of targets. Then resolve the crit hit proc where if crit the stealth buff cannot apply over buff immunity.

So unless i am missing something having buff immunity and stealth should be mutually exclusive after b2 does his special?

Same thing happens with Jka and i suppose others who can apply buff immunity.

So is this a bug or is it WAI