That wasn't the issue. Fujitsu, who operated the system, had access to post office branch systems with full access, but this was denied. The postmasters were prosecuted individually without good enough representation. The 2019 judgement is a good read about how the prosecutions happened and how the evidence was presented [1]. Really, formal methods and bugs were not issues, this was a system with humans in and someone decided that some of the humans should be blamed for issues, because the balance of power let them.

[1] https://www.judiciary.uk/wp-content/uploads/2019/12/bates-v-...