[Thorpy]

There have been a few threads on this in recent weeks.
I don't think there is a problem if you did the ESTA before the requirement change.
I seem to recall people using "old" ESTA's no problems.
I've not heard of any actual issues with anybody having issues due to a lack of both names. It's more people scared they might hit issues.
Most people that did first name only after the change because they were unaware of the change and were just filling is as per previous ESTAs have been approved as they expected. However the common path seems to be to resubmit with middle names included just to be on the safe side. I can't remember anyone confirming they did first names only after the change, left it as was and confirmed they got in without issue.

If you dig out the old threads or somebody posts links, you can get a better picture as you will undoubtedly pay more attention than I did. My key takeaway was to do ours correctly first time