It's just the example, you must use $MFORM_2$, $MFORM_3$, $MFORM_10$ or whatever you get after you create a form. The number does not matter, just use the code you get.
same thing happens in page editor if you delete a page you can not go back to that page count Why ?
This is how the system works, previous page IDs cannot be assigned to new pages. Does it really matter? That's not the problem in your case.
Could you record a video of your actions? You must be doing smth wrong but I can't guess what exactly if you are following the steps I've described.
I'm not active on the forum anymore. Please contact other forum staff.