That bug has been described on https://launchpad.net/ubuntu/+source/epiphany-browser/+bug/81056

"Many, but not all, webpages, no longer scroll correctly with Page Up and Page Down keys. Instead, pressing one of the keys causes the window to flicker very briefly, and then the window is re-drawn at the original point, instead of one screen up or down.

http://puzzling.org/logs/tech/2007/January/ is one example, for me. Page Down seems to select the next hyperlink, rather than the next screen of information, but imperfectly. Up and down keys have similar effects.

Scrolling up or down by dragging the scroll bar or using the scroll wheel on my mouse works correctly.
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> Do you use the caret mode (F7)?
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Yes, I was using the caret browsing mode by accident. http://puzzling.org/logs/tech/2007/January/ seems impossible to navigate in caret mode, the cursor will never proceed below the dark orange navigation bar near the top.

Removing the word 'correctly' from the bug title, as I don't have a good sense of how caret browsing is meant to work But it seems unlikely that it should work as it does on the given link: I can't find a set of arrow keys that will let me view the majority of the page."