If you have a dependency to a subtask, if its parent task (summary task) is expanded, the Timeline view draws a dependency line:
However, if the summary task is collapsed and the subtask is not displayed in the Timeline view, the dependency line is not shown:
Clarity 16.3.0
This is working as per the intended design.
If you would like the Timeline view to work differently, you can propose it as a product enhancement:
Alternatively, you can use the PPM Gantt functionality as it displays that it has a dependency: