Since commit 9a54df8b ("Port away from deprecated QXmlInputSource"), syndication changes the indentation of lines that start with a tag in the html content of a feed item. That doesn't matter in most places, but it's a problem where whitespace should be preserved, like in a <pre> block or a tag with style="white-space:pre". As a result, after upgrading from frameworks 5.77 to 5.80, newly fetched articles of a feed that regularly uses pre-formatted content with additional markup are shown with wrongly indented lines in akregator, while old, archived articles still render correctly. Reverting the commit mentioned above fixes the problem. A small made-up example to illustrate the problem: If an article contains <pre> line 1 <b>line</b> 2 line 3 </pre> then several spaces will be added in front of line 2, which is wrong. The offending commit should be reverted and then the deprecated method may be replaced with a different solution that doesn't cause regressions.
David, the commit mentioned here is one you pushed.
A link to an affected feed would be nice to test with here :)