I'd get in touch with Falcon Contact Us - Falcon
Or make it easier on yourself by taking your frame-builder's word.
Unless the race was ground with the utmost precision and best of specialized tools, there is going to be uneven stress there.
My only guess is that the previous owner did this to accommodate larger bearings that would fit his type of headset.
It is highly-questionable, and even if done professionally, should have been fully described and verified by the person you bought the frame from.
Since there has apparently been no mention of this alteration, we can only assume it was done by a hoser.
Talk to the seller and see if he will refund some of your money.
Don't worry: There are good, British 531 and 531c forks to be gotten out there.
There is no good or sound reason to grind the crown races on a fork.