@BobC and @roger64,
Could you try to move the id to p tag and see how it is displayed.
Code:
<p id="f5"><a href="../Text/Maeshowe_0010.html#f5.1">[5]</a> There is a similar allusion to hid treasure on the wall of a rock at Berrig, in the Star valley North Throndheim County—“gull faitu nin alna nither”—They hid some gold nine ells deep in the earth.</p>
In my case, I even use <aside> tags, I'm not sure if it is an overkill but it works fine. (Calibre Viewer, Moon+ Reader, FBreader, Kindle Paperwhite (converted to azw3)
Code:
<aside id="n1" epub:type="footnote">
<p class="note"><a href="#nl1">[1]</a> footnote texts.</p>
</aside>
It looks long but I've set snippnet for it, so it's no big deal.