I think your problem may have something to do with the fact that you are using a calling a property within a properties test step as I can use this well when calling from a TestSuite or TestCase property list.
Try using:
${MyPropertyStep#stepName} or in your case ${Properties#UserID}
instead. Seems to work for me in the same circumstances as yourself. Found the answer at
http://www.soapui.org/Scripting-Propert ... nsion.html, final bullet point.
It seems that the assertion ALWAYS succeeds where it can't find the property you're looking for, its only when you're searching for a negative response that you can tell if your assertion is working well enough.
Hope that helps!
