Take a look at this screenshot. You can see the top part of the Fire Lotus tower in the dark background...

This is the loot from five waves of the fire tower. And no, I didn't miss picking something up. I covered the complete floor.
I did another five waves this morning and got TWO grey items and NOTHING ELSE. Now just like red traffic lights, I know the low drops loom large in my memory but this is really bad.
The zero drop looks like a clear bug to me.
Comments
I can believe that my roll of the dice is six sigma to the LEFT!
Once on 21-25 of endless tower and once on 26-30 of endless tower.
I don't play lotus all that much.
I can confirm, I've had a few instances of itemless 5-rounds, frequent enough to make me wonder but not so often I've thought much about it until your post. As such, I'm also stating it is happening.
1. You have to be online to start the wave
2. You can be offline while fighting but have to be online again at the end of the wave
3. Specific (named) items drop while fighting.
This tells me that the loot is determined while the wave loads up. Can't the function which calculates the loot simply have a low-end threshold which triggers a loot recalc? Seems a trivial fix to me.
Oh, and one whole loot level in endless tonight just didn't show up when I left the map. Purple sword and dcss dropped in that one too. That was cute.