Content cut off, not fully rendered in WKWebView

We're using WKWebView to display custom content in the app. It converts plain text of a specific format to some accordingly formatted HTML. One of our users experiences the issue when the content is partially blank, though the content size (number of pages) is fine. Originally we thought the HTML is broken for some reason, but the user recorded a video of the issue where it's clearly seen that the content goes in the middle of a line, so we assume it's a WKWebView rendering issue. We should mention that we can't reproduce the issue ourselves. I have some example HTML and here's a video:

echo.playshakespeare.com/bug972.mov

The problem is NOT the content, which displays fine in Safari, but doesn't display correctly in WKWebView.