I have just noticed that the "modified" link for this question does not contain "?lastactivity" parameter in homepage questions list:
When appending it manually, it's changing to
https://meta.stackexchange.com/questions/253359/list-of-communities-with-base-css-updates-completed/286876#286876 as it should, and showing the deleted answer to those who can see it.
Other questions I checked are not affected. Possible way to reproduce might be question with zero answers, where the "modified" was caused by a now-deleted answer.