That's a placebo and it's not doing anything. Bulleted lists never need two spaces at the end of each line. You only use two spaces at the end of line that are not in bulleted or numbered lists.

The bug is that Sync requires a blank line before you start the bulleted list. The Lemmy website doesn't require that.

Here is your comment with no spaces at the end of each line:

spaces after each line. Here's a test...

• Line one
• Line two
• Line three

See? It still works fine.

Now here's an example of what two spaces do.

The first sentence below has a new line after each word but no spaces. The second sentence has two spaces and a new line after each word. The spaces force a line break to be rendered.

This Is Not A List

This
Is
Not
A
List

Source of the above:

This 
Is 
Not 
A 
List

This  
Is  
Not  
A  
List

This feature of markdown was implemented to prevent text from emails and such from wrapping in funny ways when pasted into a comment (or whatever). Old emails often force line breaks after 80 columns of text and it looks goofy when viewed in a modern web browser if those line breaks are kept, so they are ignored. To preserve the line breaks, you add two spaces at the end of each line. That or you might prefer to write paragraphs with a hard wrap at some column, but other people shouldn't have to suffer that.

Adding the two spaces to lines in a bulleted list does nothing useful, but it also doesn't break it, so of course it works.