"Save Copy As" retroactively turned into "nulnulnul", session snapshot automatically re-enabled after restart
Turning off the “session snapshot/automatic backup feature” stopped my files from getting replaced with “nulnulnul”. I was careful about using “Save a Copy As” frequently, saving to another file name. After restarting, however, Notepad++ automatically re-enabled session snapshot and automatic backup, and not only was my main file transformed to “nulnulnul”, but so was the copy I had saved.
This renders Notepad++ unusable.