That a few days of including things like whole grains and potatoes in the diet means the keto-subject has no issue with the glucose challenge, leads me to ask: what actual problem?

It seems more a matter of not being able to switch fuels on a dime. There's a known delay between alpha-cell-secretion and inhibition of gluconeogenesis. Ditto re-sensitizing GLUT4 receptors (neurons can’t function with zero glucose*). There's a similar delay at the end of a fast - major reason I try not to end a fast with anything carb-heavy (and glucola then would probably give me a massive headache)

*and with zero lactate from astrocytes — which need glucose to make that