No there is no secret here for what you are doing. If you want to make a mirror image that is more realistic then I would use the track motion and not pan/crop. You will need to make two tracks with the text. Select track motion and then change the source from source Alpha to 3D and position it where you need to in relation to the other track. You will have much better control in all planes.
Just tried to duplicate what you were experiencing and I couldn’t. Mine works just fine but I will say at times I have come across an FX, track motion, pan crop function that ceased to function for some reason. Especially seems to happen on a particular track and not all tracks. Don’t ask why I have no idea. So after racking my brains the first few times I just create a new track and then copy the track into the new one. Voila it fixes itself!! It seems Vegas gets locked in an wont let go, again don’t know why it just does.
Hope this helps in the short turn, otherwise there is not secret anything in your case.
John