Hi Thomas,
When a Repositories Trigger is set to trigger on the default branch, this means that it will trigger on the default branch set in the repository settings.
The Default Branch mapping is only used when manually starting a build.
It is also used for to define the branch to build for other non-triggering repositories in the same configuration, so I can see how you might expect this to work differently.
This will need some clarification in the UI. We wouldn’t want to change the behaviour now as this may break builds for other users, but we will look into adding another option to the Trigger On drop-down which takes the branch mappings into account.