I can confirm this unfortunate behavior.
Doing some more testing this "only" seems to happen where there is the backupfile filename~ present. If there is no filename~ present emacs does seem to leave the original file un-truncated.
I can confirm this unfortunate behavior.
Doing some more testing this "only" seems to happen where there is the backupfile filename~ present. If there is no filename~ present emacs does seem to leave the original file un-truncated.