I can confirm this behavior, the droids that start the event cast a calibration chanel very quickly and if you miss to interrupt it adds an stack of a buff that reduces damage received, once it gets 8 stacks it becomes inmune.
As a guildie said, looks like a mistake with versions control software and the new nightmare mode stealthy got into live server.
BTW, awesome job with TfB operation its the best so far, GRATS
I was wondering about this too. The patch notes mentioned changes to specific mechanics in EC, but it was right after a SM bullet (so I assumed the mechanic changes were to SM only). Did they dumb down EC HM? Is that the game plan? Do not give us a genuine NiM EC but just water down the first two so 1.3 EC HM is the new NiM?