The good folks at Duvel Moortgat have lent me some clarity
I did everything right for 6 weeks leading up to that, my HR was exactly where it should be for the whole ride and, by and large, I was riding as well as I did last year until the last km. Can't get too upset, especially given the quality of the field (I think catching a few people early gave me a real boost last time!).
Good thing about this time of year is that there's always another race pretty soon, so just got to keep doing the little things until then!