The problem is in backup_lines(). If openfile->current isn't at the top
of the paragraph to be justified, it's skipped over when backing up the
lines (so that the line after the last line of a paragraph isn't counted
when the paragraph is at least one line long and there's a magicline).
This means that the mark's beginning line is never set to the right
place in the newly copied lines, and the place it points to in the
backed-up lines is destroyed as soon as you don't unjustify.
And the patch to fix this problem in 2.0.2 is attached.