I have never testing using a record ID# as a Key field as part of a formula field.
If you want to test that in a copy fo your app, you can type the API
you would get to the tale and then observe the URL. Then after the ? in the URL you would erase what is there and type
act=API_SetKeyField&fid=99
where 99 is the field ID of the formula field.
So..
1. i have never tested using the [Record ID#] in a formula as the Key field.
2. I would test this in a copy of the app first.
During this process disable the need for Application Tokens in your app.