When it comes to fiber in particular, I think what can confuse people is the difference between water soluble fibers like oatmeal, and extremely insoluble "macrofibers" like raw green asparagus, leafy greens, orange pith etc. The former is OK for some, the latter is no-go for everyone AFAIU.
Another curveball is nearly underbaked dough, like you'll find in a lot of commercial bread and rolls, even if it has zero fiber. If you can knead it back into dough with your hands, it might just form a nice lump in your gut as well.
I have the same thing as you with corn, and also with nuts unless eaten with lots of liquids. I think it goes back to the possibility of forming tough paste in the gut.