Time was, you could go through a whole box of Generations before you'd find a good one*; these days you can't do that because of Health & Safety, which is fair enough, but no good when it comes to buying whistles! Try Clarkes, which can be tweaked, but are in any case less prone to anomalous harmonics such as you describe; and an old Carkes C is worth it's weight in sherbet, but not much use in a session where everyone's playing in D. Never got round to any of the expensive whistles because I spend all my pocket money on expensive Jew's Harps, but my favourite standard D remains my 1983 Faedog that came through in the first batch (allegedly) & it still plays beautifully - & I wouldn't part with my old Generation Bb bought from Rothbury Festival in 1984 for all the tea in Wigan.

*My standard test for a good Generation is if it can carry the old Abbots Bromley Horn Dance melody (aka The Wheelwright's Tune) without cracking up in the top end.