I found that setting hopping step as FWH clears boil time (makes the field read-only also) and sets IBU contributed as 0 for this step. Is it really supposed to work like this?
FWH additions are converted to a 20 minute addition internally inside the calculator. Here is a thread that talks more about how we addressed it: viewtopic.php?f=3&t=837&p=5247&hilit=fwh#p5232
The IBUs would be zero if you had no fermentables in the boil, or no amount specified.