I understand that Pega is taking the work urgency as a basis for calculating the assignment urgency. So whenever an assignment is created or updated, the work urgency (.pxUrgencyWork Work-) gets copied to the assignment (.pxUrgencyWork Assign-), which will trigger the declare expression to calculate the assignment urgency (.pxUrgencyAssign).

What I now not understand is that there is no automatic update of the assignment urgency whenever the work urgency changes.

For instance, when a case level SLA is increasing the work urgency, why is then not automatically the assignment urgency being increased?

We used the following approach to assure this:

We have implemented an Declare OnChange watching .pxUrgencyWork. When this is changing, we call Pega's OOTB activity "resynchAssignmentUrgency".

Working perfectly and should be in my opinion completely Pega OOTB.