WVA has himself stated in several inscyd propaganda pieces that he is a sugar burner, or conversely fat metabolism limited. The idea being that if substantial amount of glycogen is burnt already during the build up to the finale, top end is impaired as there's eventually little to burn. Those pieces of course try to generate hype for the testing protocol and vlamax theory, which is alleged to uniquely (sic) address issues such as this one.
Without commenting on the inscyd model's general validity, which I personally doubt, the degradation of anaerobic and sprint performance after, say, 5000kj in races does fit the story quite well. In shorter races he does insane feats such as the TDF stage four, but is found lacking in longer ones.
Shoveling 120g or so of sugar into the gut every hour of the race may help, but perhaps punch after a long hard race just is his arch weakness