The pxSessionTimer is only meant for use within a full end user portal. So when you are imbedding PRPC content into another application using createNewWork or display the code hooks for the pxSessionTimer are not there. I have seen it work when displaying a portal with IAC but that is just because my test case is displaying a portal that has a pxSessionTimer already in it.
I would not use a pxSessionTimer with IAC anyway as we are being imbedded into a top level application. The top level application would have the same requirement so I would use the PegaE_onload event in the gadget definition to trigger resetting the parent application timer. The PegaE_onload will fire whenever a harness is loaded, so as the user is progressing through an embedded flow or set of screens you can script for resetting the partent application timer.