Unable to get different cover size

I’m in the process of building a custom archive page for the reviews and would like to add a thumbnail of the cover for the entries.

Trying to use the \Rcno_Template_Tags::get_the_rcno_book_cover method, I realised it doesn’t return the image size I asked for.

Looking at the code, I found the problem here:
https://plugins.trac.wordpress.org/browser/recencio-book-reviews/trunk/public/class-rcno-template-tags.php?rev=2678712#L373

esc_attr( $og_book_src ?: $book_src ) will always return the original URL ($og_book_src), not the custom size one ($book_src).