Found the culprit: creepy
Space Invaders !!
Somehow in your version of Notepad a space was inserted following the line-numbers and before [CR/LF] (or
'newline'). In the English original these spaces were not there. In the image below you will see what I mean. Left is with the spaces (which should not be there); Right is without spaces (like the English original), which is shown as expected.
(Note: in fact a space was/is inserted before virtually every 'newline-code' not only those of the linenumbers.)
That being said ...
... if you select "Use new subtitle parser/renderer" under
(rightclick >) Options > Preferences > Subtitles [Subtitle properties]
you will see those (extra) spaces don't influence a correct display of subtitles.
However when you don't use this "new" option, then the "old subs-engine" apparently supposes it has to show (its max) three lines, the second being an empty line and the third being the line-number (if followed by a space before the [Enter]).
P.S.:
While testing it appeared to me that whereas the old subtitle-engine allowed a maximum of 3 lines (which IMHO is more than sufficient for normal subtitles), the new subtitle engine can almost just as well be used as a kind of telecue or teleprompter (provided the number of lines does not cause the top lines to be displayed over a black upper border -if present-
(where the picture isn't refreshed)).