Cannot change setting when creating a new photoshop document

Hello all,

Something curious going on. One of those things where everything is fine. You wake up the next day, you've changed nothing and something's wrong. Maybe my cat stood on a function key.

I cannot change the setting for whether I want to work in pixels, millimetres or other when creating a new photoshop document (left), but can with an existing document (right). I can change size and resolution but not the format.

I've tried the different tabs at the top, clicked on various custom options and gone to preference options but can find nothing to get this to work.

Every document I create is unique. Generally, I don'ty use a present. I want to manually type in size, resolution and whether I'm working in pixels or another format.