Arbiter of Ash - question about changing the difficulty level - need help.

At his gate where you put the keys in there’ll be an option to fight his harder variant, theres only 2 arbiter difficulties. Allocating points on atlas tree does nothing for him

Now there are only two versions of arbiter of ash : normal and Uber. Since you have all points in boss atlas passive tree, you have access to the Uber version via crisis fragments. Iirc dying during the Uber arbiter let's you retry it but in the normal version. Personally I found a prism of belief in a normal arbiter (after dying once vs Uber arbiter) so I guess this is bad luck?

Interesting fact: woman can now swim topless at swimming pools in Berlin.

Once your boss atlas tree is fully allocated (like in your picture) the citadel bosses will drop improved fragments (red instead of grey) that you can use to fight Uber arbiter instead of the normal arbiter.

You are just going over the drop rate, that's the short response.