This .pkm file worked for me. You could try it out and see if it works for you too.
It won't change back into normal form because it doesn't have the multitype ability.
Arceus Unknown.pkm
I was fumbling around with this stuff and I can confirm that, if you change Arceus ability to, lets say Pressure like many other legendaries, it stays in Cursed form. But I belive that it is still Normal type. Atleast Judgement is.