This idea has been merged into another idea. To comment or vote on this idea, please visit A-I-9228 Enable release records to have a bottom section for custom fields.
The layout customisation for Releases differs from that for features in that it does not have the "top section" and "bottom section" options. I have some custom fields set up on my releases which end up appearing in the top section when i'd really prefer them to sit below the "Theme" area.