Step to reproduce:

1. Create a new Multibranch Pipeline
2. Choose "Bitbucket" in Branch Sources, so it uses Bitbucket Branch Source plugin
3. Fill in data of a Mercurial repoistory
4. Save and Build
5. Build status (both build begin and complete) is not reflected on repo page at Bitbucket website

If the repository is git, build status is showed correctly, and console log has this line twice:

[Bitbucket] Notifying commit build result 

But there is no this line in the console log of the Mercurial pipeline.