Sharepoint site's customized column become 'Read-Only'

We have a SharePoint Intranet that has been in active use for 14+ months. I have a customized column Expiration Date for News Expiry.

Recently, all custom columns added to Site Pages (Page Details) or News pages have become read-only immediately after clicking Save.

Key points:
- Issue occurs for ALL column types (Date, Date & Time, Text, etc.)
- Fields are editable for the first time.
- As soon as Save is clicked and page or news came to draft status and the field become read-only.
- Happens for both Draft and Published pages.
- Editing in Site Pages library Grid view still works.
- This behavior started recently without any configuration changes.

Has anyone else experienced this?