Ok, I have found the problem.
At some point in the history of my site I installed a theme which inserted a lot of dummy data. This made pages that have the “Lorem Ipsum” text in the post_excerpt. A year later, I switched to Astra Pro theme and Elementor, which do NOT show the Excerpt editor. I then customised the existing pages and forgot about it, until I noticed it in the search results.
I was able to find it by running this SQL on MySQL:
SELECT * FROM wp_posts WHERE post_excerpt like '%lorem%'
I fixed it like this (run this at your own risk and backup first).
UPDATE wp_posts
SET post_excerpt=""
WHERE post_excerpt like '%lorem%'
Then I cleared all caches and checked View Source and it’s gone!