My problem with the February 2012 nav data update on the 296 was similar in some respects (no airways shown) but different in that all the lower class D airspace was also missing. The March 2012 update (loaded gratis by Garmin) cured the problem.
But like you, the source of the problem was the update supplied by Jeppeson.
FWIW, Garmin UK behaved incredibly well and solved the problem as well as showing a real will to help (in spite of the 296 being 6 years old at least).
My experience of Garmin's product support has been second to none.
