An updated test script is ready for review that should be more reliable to servers that may not always be able to guarantee the requested/revised sampling rate.
3 changes have been done, each one by itself should resolve the issue.
Because the test script does not validate any timings, the KeepAliveCount of the subscription has been increased from 1 to 3. If the server was not able to sample a new value within one publishing interval, the server does not need to provide a KeepAlive which cause the test script to fail.
In addition the test script waits one PublishingInterval/SamplingInterval (whatever is slower) between the write requests.
Finally the test script issues Publish up to 10 times to get the intitiated data change before aborting the test.

reviewed in call, agreed to changes and closed issue