It certainly got better in terms of success rate, but this morning in GW node 2, Ewok Elder (7* level 71) did not remove Healing Immunity cast by a level ~60 Sid.
Can confirm this, I think it is related to the amount of debuffs on your team, since the more debuffs on your team, the more still remains after his heal. If all chars have 2-3 debuts, only 1-2 will be removed/char, with variations.
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It's true that with multiple debuffs, for example, 2-3 dots along with speed down, he will remove only 1 debuff tops when it fails.