Update: we have determined that whenever the HR strap is active and the the iBike is connected to the wireless mount, the iBike does show both average and max HR values as desired.
However, if the HR strap is "Off" or if the iBike is removed from the wireless mount (effectively turning the HR strap off) then average/max HR won't be shown in the bottom window.
The workaround is simple: at the end of your ride, check your average/max HR before removing the iBike from your mount. Or, if you want to see stats later on, attach the iBike to your wireless mount and attach your HR strap so that the iBike knows the HR strap is awake. That will allow you to see average/max HR values.
We will change things in a future FW release so that average/max HR will be shown even when the iBike is not receiving an HR signal.