Currently, the Webhook payload is limited to 256 KB, and the trigger silently fails without an error/notification.
https://support.atlassian.com/bitbucket-cloud/docs/troubleshoot-webhooks/

Use-cases

When users merge a Pull Request, Bitbucket Cloud will include all the commit messages from the Source branch as the new merge commit's message by default.

• If the Source branch has large commit history, the commit messages will be combined and caused the webhook to fail due to a large payload.
• This prevents CI/CD such as Pipelines from triggering Build for large Pull Request
  https://jira.atlassian.com/browse/BCLOUD-21021

Suggestion

It will be best that Bitbucket Cloud support a larger payload