This is in the Twisted Caverns mission? That zone is incredibly laggy. The game client is displaying Zero Damage hits because the server isn't giving it data back. I had the same issue and then eventually things started working but still with a lot if lag and stuttering.
"We have always been at war with Dread Vault" ~ Little Brother
It's in several of the new areas, and I considered that it was due to lag and hopefully will be fixed once lag is fixed, but it is being logged in combat and so is not just a visual bug. Mobs are also doing 0 damage to players. This is across several characters, several people, and several areas.
It happens all the time to me in all Uprising locations: First attack is zero dmg to opponents, with very few exceptions, even when there is no apparent lag.
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https://photos.app.goo.gl/br2o4RK5ZEWiZKje9
More instances of it happening - seems to happen all the time, not really seeing it anywhere else.