Copy and paste text from a pdf inadvertently added a <i> to some text. Normally I would use <em>
This tag affected all the text below it even if it was in different sections.
It can happen that copied text from any source (PDFs, websites, …) contains inline styles and html tags. If you notice this when pasting, you should clean it up accordingly. We are not responsible for the source (in your case the PDF), nor for what your clipboard contains (unwanted tags).
If you paste the text into Gutenberg you will probably get the same result, completely independent of Bricks or the rich text editor.