com.apple.quarantine extended attribute gets re-added each time I open the file
You need to add the direct path to the file  in your command to remove it permanently:

xattr -d com.apple.quarantine /complete/path/to/tasks.taskpaper


If this fails, there may be multiple instances. Attempt to globally remove it amongst all versions:

find . -iname '*.taskpaper' -print0 | xargs -0 xattr -d com.apple.quarantine