Having problems with workflow rule and an action not being fired

[Snoopy]

Hi everybody,
I have the following problem and run out of ideas what the root cause could be.
I defined a workflow rule for my table that is based on a row-level action triggered by a static field. When I set the field to invoke the action, the workflow rule test detects it, but nothing happens. When I execute it from within the tester, I get a success message, but nothing happens, either. Only when I manually fire the action from the detailed view, I finally get the result that I want.
And when I add an email send action to the workflow rule, that email is also sent out. So the Workflow rule does detect the trigger event correctly.
Is this a bug, or still a lack of understanding on my side ?

Besides, the RecordWarning field in the test report is flagged, but I suppose that is due to the email warning when the app is not yet deployed ?

Any idea or suggestion ? Thx.