"Osipov, Michael (IN IT IN)" [off-list ref] writes:

An oversight or bug?

Neither, i.e. WAI, I would say.

The configuration variable log.date is about "git log" and its
documentation makes no promises how "git bisect" may or may not be
affected.

Having said that, I think it is not unreasonable for you to make it
a feature request to add bisect.dateformat or whatever.  The only
interesting part from the output being the exact commit object name
the problem bisects to, I personally would not see it a high priority
feature request, though.

Thanks.