Currently the default description on Pull Requests feature under Repository Settings will overwrite the description generated while a new PR is created.

Kindly provide an option to keep both
ie,

1. The default description from repo settings
2. The auto generated description during PR creation.

Latest update from Atlassian 2023-07-20

We have added a capability that lets you substitute a list of commit messages by using the variable {{commit_messages}} in a default pull request description template. Docs are here: https://support.atlassian.com/bitbucket-cloud/docs/pull-request-and-merge-settings/