Last night I had a boilover 20 min into a 90 min boil, and a good portion of hops (pellets) boiled over and out.
At that stage, are the hop oils already in the wort, or are they still being extracted from the pellets? Will the wort continue to bitter? Does the loss of those hops decrease bittering?
With pellet hops, the smashing process bursts the lupulin glands and makes the materials available.
I believe that you will get most of the essential acids and oils off the hop material within the first two minutes or so, maybe faster.
From your description, the loss of the plant material should have no real impact on your bittering.