Unwanted double line breaks

When I export my WIP (fiction) to epub, it looks perfectly fine in Calibre - each new paragraph starts with a line break and indent. It also looks like it should in Apple Books. But when I transfer it to my ebook reader (Letto), I get double line breaks and no indents for new paragraphs. Basically, it looks like this post, rather than a novel.

I have previously compiled to epub and transferred it to the reader without this problem, so I’m guessing there’s something going on in the compile that’s different - but I’m at a loss trying to find out what.

Please advise. Thank you.