Law.com has this on the DOJ apologizing to the District Court for the District of Columbia for attempting to withhold names and email addresses of Trump transition members that are already public.
To those who don't regularly practice FOIA litigation, its quite easy to see how this occurred. The US Attorney's for the District of Columbia represented the agency in this suit; the US Attorney's Office has multiple FOIA lawsuits and relies on agency counsel to provide answers to various filings by the plaintiff. My guess (I have no knowledge that this happened) is that agency counsel and FOIA staff told its counsel that this was non-public (with very little research into the matter).
There is a large uptick in FOIA litigation, most of it going to the U.S. Attorney's Office - this is the result of the unintended consequences of an administration that doesn't believe in transparency and has a number of individuals in charge of agencies who do things that cause the public to file FOIA requests to find out what "the government is up to" as Congress has punted on its oversight responsibilities (because they are afraid of a nasty tweet). I would expect more of these situations for the next 31 months.