In the last couple of weeks I have had several occasions where I have tanked a world boss, alongside a handful (less than 4) DDs, where I am taunting, doing some AoE healing, and generally doing what tanks do - and not been able to loot the dead boss.
It does not happen every time and I can find no pattern in it. For example, today I turned up to a Galen WB who was at 10%. Taunted, hit a couple of times, there were 4 DDs there already. I got loot. Ten minutes later, I tanked a Deadlands WB from 100%, there were 3 DDs there, I got no loot.