Not sure of the cause, but have been having cut-and-paste issues as well directly using the mouse. The editor and browser have upload and save-as functions that do not seem to truncate. If I am grabbing a section of Discourse I tend to use TIU browsers as the easiest method lately. If you have lynx or md3 or links or … you can do things like
lynx -dump https://fortran-lang.discourse.group/t/new-post-editor-panel/9986 > discourse.txt
from the command line in GNU/Linux or WSL or CygWin.