It was considered a feature, not a bug - the guidance/avionics software for the main computers was extremely complex with millions of lines of code, absolutely no way to guarantee there wasn't a showstopper bug lurking there somewhere. All four main computers ran the same code developed by IBM; the fifth backup computer's code was written entirely independently IIRC by the Rockwell team.
The most likely cause of a "majority-rule" fault was considered to be a hardware issue, but a 2-2 deadlock was more likely to be a software problem, and in that situation you simply can't trust the four main computer's software to do anything correctly anymore so you bring up the fifth computer to run the critical sections using an independent codebase.
The probability of both a showstopper bug in the main code and another one in the backup causing a loss of vehicle accident was considered to be so remote as to be an eventuality that wasn't addressable by any reasonable amount of engineering effort. AFAIK a 2-2 split never occurred in practice