Certain dyes are either impossible or possible to mix in a way that they shouldn't be.
Red dye is unmixable, when it should be mixable through magenta and yellow.
Blue dye is unmixable, when it should be mixed from cyan and magenta.
Brown dye is unmixable, when it should be mixed through darkening orange with black.
Green dye is unmixable, when it should me bixable through cyan and yellow.
Black dye is unmixable, when it should be mixed through yellow, magenta, and cyan.
Magenta and cyan dyes are mixable, despite being primary colors.
That's all I could determine, though I may have missed some things.
Feedback site: https://feedback.minecraft.net/hc/en-us