Custom approver field made in workflow not available in work type fields list

[Merijn]

Hi,

I am trying to make different approver fields for different types of approvals that our work-item requires. 
When I enable "Approval" in the settings of a status in our workflow, and then click the settings of the approval workflow, there are two options: "Select preset approvers" or "Select or name the approver field".
In my case I need it to be a configurable field, as the approver depends on the specific work item.
In the dropdown for the Select or name the approver field, none of my custom user picker fields are showing up. You can also create a new field by typing the name in the dropdown and clicking the "Create "your name"" which it accepts in the workflow editor, but that approver field does not show up in my work item and is also not showing up in the list of fields that can be added to the item.

I have a bunch of approver fields that I created from within the status in the workflow editor showing up in that dropdown, which don't seem to exist anywhere else in our Jira environment.

CLicking the "Learn more about setting up approvals" link in the status settings leads to a "Error 404", so I am trying it here.

How is this feature supposed to work?

Thanks!