I think I found a small bug.  We are using Forms 12.0.2503.10378

I was creating a new form and had just dropped 2 fields onto the form.  I was going to use _(Current User) as the column match and look up a couple pieces of info.  The "add target" (+) would only display if I added 3 fields to the form.  With only 2 fields the plus wouldn't display.  I know it's a very small thing but set me back a few minutes trying to figure it out.