I find this to be an honest mistake that was quickly corrected, and it should be acceptable to do so. We shouldn't have a rule that prevents a board member from changing an obviously incorrect vote, but I agree that we should not change rules mid-process. In this case, however, we may not have to.
According to voting rule #3, we require a reason to be given for a choice. Granted it does not say if the reason has to make any sense, but I presume that is somewhat implied. Or am I overinterpreting the voting rules and any random, illogical reason goes?
Certainly, it can get murky, if we suddenly have to evaluate whether or not given reasons make sense or not, but we should at least reject votes with obviously bad reasons. From the original vote it was clear that something was wrong, as the argument for option B was: "Easier for vulnerability scanners in 6-digit fixed length". Option B is neither a fixed length nor 6 digits.
My thoughts on this are that it would be reasonable to consider the first cast vote as invalid due to the erroneous reason given. The second vote has valid reasons that match the options and should be considered valid.