@Doitsu,
So the exact same version of ADE 4.5.11.187303 on Windows works fine for epub2 with 100% but not for epub3? Or did I misread and you tested with a slightly different build number?
Code:
<div style="height:100vh; text-align: center; padding: 0pt; margin: 0pt;">
<svg xmlns="http://www.w3.org/2000/svg" height="100%" width="100%" version="1.1" preserveAspectRatio="xMidYMid meet" xmlns:xlink="http://www.w3.org/1999/xlink" viewBox="0 0 1080 1430"><image width="1080" xlink:href="../Images/cover.jpg" height="1430"/></svg>
</div>
And your version of the workaround of setting the height on the parent div really makes sense as its svg child should use 100% of that height.
Sigil does have epub version specific code for the cover so adding that workaround to the epub3 version is easily doable.