Dragon Soul issues.

[Islingr]

Also Spine HC can be pulled without first 6 bosses down on hc and hc madness can be pulled without spine HC

well if this reddit thread is anything to go by, you indeed do need to kill all bosses on heroic previous to the last boss

[romka199]

What? You can switch difficulties any time you want lol, even if u do first 6 normal you can switch hc and do rest on hc, thats how its supposed to be i bellive

You actually cant. You need to kill all previous bosses on Heroic if u wanna do spine and madness on heroic also. Cant do some bosses on normal and then spine heroic.