This behavior has been tested against Logseq 0.10.14 for Windows.
Paragraph breaks
You can use HTML P elements to create paragraphs in a table cell. Every <p> tag should be prefixed with space and have a matching </p> tag.
Within a P element, be careful not to use P tags. Even backquoted P tags will be interpreted within a P element.
Line breaks
You can use HTML BR elements to create line breaks, but only within a P element. Outside a P element it will be rendered literally.