If you apply a new paragraph style it will apply to all the paragraphs that are wholly or partly included in the highlighted text. And a paragraph only ends (as far as LO is concerned) when a paragraph end mark is inserted by pressing the Enter key.
When I come across your problem it is usually because the text includes blank lines which give the appearance of separate paragraphs, while LO will apply the formatting to the following text up to the next paragraph end mark.
Set Writer to display at least some formatting marks including particularly “Paragraph end” “Breaks” and “Non breaking spaces”. To do this open the “Tools” menu and select “Options” > “LibreOffice Writer”>”Formatting Aids” for a list of what you can have displayed. Some people find the marks a bit off-putting but you should soon get used to them.