I think they turn off radar when you do. Get out in front of them in an aircraft with RWR and turn your radar off and on and see if you get anything showing up from behind you (I also may be thinking of another sim). I have given my wingman and my element attack orders from 15 to 20 miles in Vietnam F-4's and they engaged with Sparrows.
If I'm reading your comments correctly, you made a copy of the program files prior to reinstalling Windows 10, then copied that back into the Programs folder after the reinstall. Then you re-installed a clean copy of SF2. Is that correct? When you copied the program files did you also copy the Users...Saved Games files and copy them back into their folder? You may be having a conflict between the old files and the new files if you re-installed SF2. I'd rem out the old files (or temporarily put them in another folder) and try re-installing a clean copy of SF2 and see if it runs. Then copy whatever you need from the old files into the new Saved Games folder.