The setting for strict line breaks only has to do with what is understood as the beginning of a new paragraph. In normal markdown, there has to be an empty line between two paragraphs - if you just enter one single line break, markdown doesn’t interpret it as a new paragraph.
There are two possible solutions to your problem:
The first one is with css - you can define more space between two paragraphs. This setting, however, then would apply to every paragraph.
If you just want to insert more space between two lines/paragraphs from time to time, the only solution is to use html. <br> is the html syntax for “line break”. Try to include several of these and see how it looks in preview mode!