Exit Required Field in workflow behaving as Warning & not Error

We have configured a workflow on a subclass of Activities class, where we have put one of the page 3 attributes of that subclass as an exit criteria for status Pending on that workflow. But surprisingly, when we try to move the object from Pending to next status (which is of type Review), by keeping that Page 3 attribute blank, a pop up is opening which is showing a Warning about the error & not an Error, hence allowing to move the workflow further without any prevention. What may be reason behind this ? Is this an expected behavior or