That's not how any version of WinRAR has ever worked for me. I've tried 4.x and 5.x today, both with default settings and split archives they created themselves, and they always truncate when opening (not extracting) a later part.
Make sure there are some small files at the start of the archive so that the part you open doesn't start with part of a large file that spans from the start to the opened part. (Although even then, WinRAR adds some arrows after the filename to indicate parts of the file are in previous parts of the archive).
TBH, I don't really understand why this is such a problem either way, especially with modern RAR split archives that have the .part01.rar as the first file and never make you hunt for it like the old .rXX naming convention.