I have updated to version 5.13.1 (build 8265). Now when I connect to remote linux server, open nagios configuration file with around 650 lines of code, select "Ctrl+A" then "Ctrl+C" and paste in a local notepad only 170 to 180 lines get copied. Tried with multiple files.
Thanks for your report. What version of WinSCP were you using previously? Are you opening the file in WinSCP internal editor? Do you have the problem with that specific file only? Or with other files too?