My understanding is that rdesktop has a main loop that works for input events, and while rdesktop can recognize that the ThinLinc session has changed size, the actual reconnect process is not necessarily initiated until rdesktop processes the next input event. On normal desktops, this isn't such a big deal since you move the mouse around quite often. On tablets where you don't drag pointers around you don't have as many input events, making it seem like rdesktop won't resize along with the session.
*** This bug has been marked as a duplicate of bug 4480 ***