There are several reasons causing this issue:

1. The wrong variable name as you said: $row should be replaced by $item

2. The getListQuery method of TagsModelTags class doesn't return author, author_email, fields, so it will throw warning in other line code as well : https://github.com/joomla/joomla-cms/blob/staging/components/com_tags/views/tags/view.feed.php#L56

3. The code in this line will also throw warning as there is no displayDate returned for each item https://github.com/joomla/joomla-cms/blob/staging/components/com_tags/views/tags/view.feed.php#L57

I can make a PR to fix these issues (it is quite easy). However, do you know how we can see there warnings (so that I can give testing instructions in the pull require) ? Is there an easy way to show these warnings ?

OK. I went ahead to make pull request to fix this issue #4651 (and several issues I found as well). Please help testing it :)

Why does this not have the PR-staging tag?

Sorry. I deleted the original repo which I made the PR. I think that's the reason :(.

Since we have #4651 to fix this, I'm going to close this issue report.

Set to "closed" on behalf of @Hackwar by The JTracker Application at issues.joomla.org/joomla-cms/4623