drupal/prototype:dev-3.x returns old version

3.x-dev is the https://www.drupal.org/project/prototype/releases/8.x-3.x-dev → release. Version numbers for Composer drop the 8.x- API compatibility prefix. That release is packaged from the 8.x-3.x branch. 15ebbd47787c19907649fa6c55497931411abfb5 is the most recent commit on that branch.

I recommend the maintainers merge 3.x into 8.x-3.x and delete the 3.x branch to avoid further confusion.

We have removed the 3.x branch and are in the process of addressing the support needs for the 8.x-3.x with a new tagged release