I think the dumbed down version is a different API — GtkColorChooserDialog (new in GTK+3) — while Geany only uses the traditional GtkColorSelectionDialog (deprecated in GTK+3).
There have been reports of the dumb one appearing in Geany, so thats not it, anyhow so long as the sensible one continues to be available I don't care.
By the way, when you tried to reproduce this, did you select the color by clicking on the ring/triangle, or did you directly adjust the individual inputs like H/S/V? It could be that only the ring/triangle have enough resolution to trigger this issue.
Yes, but I may have just been lucky, after more testing it failed a couple of times, maybe 1 in 5.