Here's one time I'll disagree that the test is bogus.
I had a Hefe batch fail to convert during a very rainy spell in March many years ago. I was new to AG, and I didn't check my mash pH. 2 hrs of mashing and testing and the iodine test was always black. Added a tsp of gypsum and the iodine test showed full conversion within 15 mins. Essentially, the aquifer for our city water was "channelling" and I was mashing in basically what was basically rainwater.