Ignoring top margins immediately after an internal page break is a known quirk with Kindle readers:
https://kdp.amazon.com/community/mes...ssageID=311217
If you don't want to split the file into multiple HTML files, you can probably work around this quirk by adding a block element before the <h1> tag, e.g.
Code:
<div style="height: 1px; width: 1px"></div>
<h1> ... </h1>