Linux Mint 20.1 / Calibre 6.11
Up to fairly recently I had no problems creating popup footnotes in ebooks using examples such as the following:
For the footnote anchor:
<sup id="calledF1"><a epub_type="noteref" href="#dataF1">*</a></sup>
Which links to the following footnote (at the end of the file, for example):
<aside epub:type="footnote" id="dataF1">
<div class="footnote"><a href="#calledF1">*</a> Footnote popup text here.</div>
</aside>
And for good measure, the declaration in the Body tag:
"xmlns:epub="http://www.idpf.org/2007/ops"
The declaration at the start of each html file is
<?xml version='1.0' encoding='utf-8'?>
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
This always worked in the past, but for the past few versions the popups created in the ebook editor have stopped working and become linked popups only.
However popups created in earlier versions of Calibre still work correctly, as do popups in purchased ebooks.