If you don't respect the compulsory spacing line,
the subtitles will appear on the video with a "timecode hiccup".
The time and text of the second block appears indeed on the video,
as a continuum of the 1st, without spacing line.
The compulsory space line should be an empty line.
A similar "timecode hiccup" happens otherwise.
The time of the second block appears indeed on the video.
Due to the presence of an unvisible space character,
the second block is considered a continuum of the 1st
with an apparent spacing line in this case.