The Immobilize affix (e.g. cleanses and prevents Snares/Immobilizes) is bugged on Live and does not actually prevent them.
This was present on the PTS as well, where the Immobilize affix would fail to grant immunity if the Banner was activated before a combat instance began (in that case, a duel). But it would be granted if the Banner was activated after the combat instance had already began.
However, last night I played in Cyro with the Immobilize affix on and I was catching Immobilize after Immobilize while the Banner was active. I would toggle the Banner on and off and it would cleanse my current Immobilize but inevitably I would catch additional Immobilizes soon afterward.
Contrast this to solo PvE where I tanked mobs in Spellscar for twenty minutes without catching any Snares or Immobilizes. And those enemies spam them. So the affix was working in that scenario. And this was with activating the Banner prior to combat as well as activating it during combat. So the issue present in duels was not present in that sample.
Suffice to say... something is not working as intended with this affix. Overall, Banner is extremely buggy across several different affixes.
Ideas:
1. There is a bugged interaction with Battle Spirit (I have only had this affix fail during PvP, either duels or group play in Cyro)
2. There is a bugged interaction with other players entering the Banner radius (a la the currently recognize Arcanist class mastery bug)
Both of those could be true. But either way, the Immobilize affix is NOT working consistently and is NOT a reliable means of preventing Snares/Immobilizes in PvP.