In Fargo, why did the police ignore that Chumph was duct-taped to the chair?

We can't say that the police ignored this fact. Of course they would have taken notice and known that someone else was involved. It's just that this part is not discussed in further story. If you look at the complete story from the beginning, there were a series of bizarre events which baffled the police, but the police is shown kind of helpless and clueless, unable to crack the cases. I mean it is not possible to show each and everything in detail. So I guess the incident you mentioned was just another one in the series of events happening in the town.

And to answer you question specifically, I would say the police did not ignore the fact or forgot about the case. They were just unable to figure out "who & why" aspects of the case.

Actually, if you listen, as this scene concludes, and we transition to Malvo, listening over the police scanner from his car, you can hear a police officer say "looks like suicide by cop."

... tho, that'd make a nifty trick with the duct tape, I agree.