Hello,

The Description field is a default field (non-custom), and the 'View Issue' screen only allows you to control the layout of custom fields in the Details section of the 'View Issue' page.
The 'View Issue' page ignores all the non-custom fields display orders on the Screen, and JIRA was designed to show the Description system field as the last one on the Details section.
This means that, even if you set a field to show below the Description, it'll be displayed above it.

If you create two custom fields (e.g. Free Text) and set one to be displayed above and the other to be displayed below the Description field, like this:
1- Custom Field 1
2- Description
3- Custom Field 2

They'll be displayed like this:
1- Custom Field 1
2- Custom Field 2
3- Description

Please let me know if you need further explanation.

Regards,
___________________
Felipe Kraemer
Atlassian Support