Our project has a subproject for internal affairs. The subproject has additional trackers enabled (e.g. "projectmanagement").
The milestone "version 2.3" belongs to the parent project and is inherited by it's subprojects.

Viewing the roadmap of the parent project it says milestone "version 2.3" has 2 tickets – but it only shows 1 ticket in the list (a ticket in tracker "defect").
The second ticket is a subproject ticket set to a tracker which is only available in the subproject. Such tickets seem not to be shown in the parent project roadmap view.

BUT, and that's the strange thing, these subproject tickets +are+ shown on the version page (e.g. @http://{redmine-uri}/versions/{ID}@ ) where the version belongs to the parent project.
There seems to be an inconsistency between the roadmap and the version view.

I would suggest to show +all+ tickets in roadmap view - even if the specific tracker is only enabled in a subproject.