Ordering posts by custom field named “date” in backend

I defined a custom field named “date” and now want to order the post list in the backend by this field.

I defined the filters to make ordering work and header is clickable and shows the order arrow icon. The problem is that the post list gets ordered by publish date instead of custom field “date”.

Do you think the problem is the name of the field, do I have to rename the field to make it work? Or is there something wrong with my code?