[stumped]

look closely at preferences. there are lots of detailed expandable preferences that affect which css things moon changes and which it leaves alone
you may have to be within an open book & then access them within the many preferences screens - they are there -somewhere !
otherwise, if it makes a mess of one particular book then edit the books styles for that book in calibre or sigil, to something that renders better
i would delete the between-sections image and replace with a simple *** [ using find replace all regex ] and I'd delete the header underline thing- which probably is also a decorative image