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
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 :)