The discussion at the time was that there are exception and thus the should, instead of changing the test case, I would propose that the specification should be cleaned up to include the exception where it is allow to not process the higher priority first (like all notifications in a prior publish did not fit in the publish then it continue with that queue).
The test case as it is defined would not encounter any of the exceptions.

Waiting for any feedback from OPC UA group, script should already cover edge cases