It's solved now.
1st select custom mode for both files. Save ---> then original file content paste in custom file & save it again.
& keep custom mode for both file. Original mode something issue.
Also, cross check, server original robots.txt , htaccess + backup files both content are same or not.
So, if everywhere same file content then its correct setup.
Face this issue on my site, so for more clarification update given here.
Hope, its help everyone.