Describe the bug
I have a release-date field with a custom date_format set to DD-MM-YYYY, so something like:

name: 'release-date'
label: 'Release date'
widget: 'datetime'
date_format: 'DD-MM-YYYY'
format: 'DD-MM-YYYY'

Then I have release-date set under sortable_fields, which gets recognized, but doesn't quite work as expected.

Expected behavior
I would expect the items to be listed considering the date_format and sorted by year first, then month, then day. Not the opposite as it seems to be happening.

Screenshots

Applicable Versions:

• Netlify CMS version: 2.10.71
• Git provider: GitHub
• OS: macOS 11.0.1
• Browser version: Brave v1.16.76

• Node.JS version: 12.17.0