Here's a test .epub that helps to demonstrate the issue:
sample1.epub.zip
The text inside <fieldset> tag is not displayed at all (not just the box outline is missing, the text inside is gone too).
<fieldset>
<p class="calibre3">Some people acquire the travel bug. Others are born with it. My affliction, if that’s what it is, went into remission for many years following my aborted expedition with Drew. It resurfaced after college with renewed fury. I desperately wanted to see the world, preferably on someone else’s dime. But how? I had no marketable skills, a stunted sense of morality, and a gloomy disposition. I decided to become a journalist.</p>
</fieldset>
It seems to work in other readers (screenshots from Apple Books and Yomu attached):
Here's a test .epub that helps to demonstrate the issue: sample1.epub.zip
The text inside
<fieldset>tag is not displayed at all (not just the box outline is missing, the text inside is gone too).It seems to work in other readers (screenshots from Apple Books and Yomu attached):