In the current implementation, the assumption seems to be that a boundary escalation event is triggered by an escalation end event inside the activity (i.e., a compound activity such as a sub process).
However, similarly to the feature that was once added to C7 (and is documented here), this doesn’t fully support all use cases, in particular that of boundary events attached to a task, such as a user task, service task, send task, etc. This is because these cases require an API that can be used to throw the escalation from within the task implementation. Currently, there is no such API for C8.
Is there another way to trigger escalations in these cases? If not, I guess this turns into a feature request