They have to produce two buckets of steam to produce power.
For me, they ARE producing two buckets of steam, except a small chunk of steam is immediately converted to power every tick, thus the display showing under 2000 mB of steam.
Doing a little more testing, if you stop a conduit from drawing all the power as its generated and let the internal buffer fill, the Output will show 320 RF/t. If you then reactivate the conduit, the display will keep showing the 320 RF/t being output, but mostly because now the steam tank has a constant over 2000 mB of steam instead of just under it.
So from a dead start, adding water and fuel to a Dyanmo WITH a Conduit or Pipe drawing all the power away immediately, it IS producing the same amount of power, only the readout is not displaying this properly. (this is why I had the Vibrant Capacitor above to check the input rate)
What is possibly happening is your not filling the fuel fast enough, or there is just enough of a pause at one point that the Steam drops below the 2,000 bucket level and the display stops at showing 0 RF/t, even if it actually is producing power.