Update: The cause of the problem is with the Route66 plugin installed on the website.

Steps to reproduce the issue

Create a tag.
Create an article, tag it with that tag.
Create a new menu item.
Choose Tags - Compact List of Tagged Items

Expected result

When you choose that menu item, you should get a compact list of only article titles displayed.

Actual result

When you choose that menu item, you get a regular list, with the whole article displayed.

System information (as much as possible)

Joomla 5.1.0
Multi-language site

Additional comments

Previously (I can't remember up to which Joomla version) this worked fine. The previously created menu items don't work, and the newly created menu items give the same result.

The similar Category lists also work correctly, it's only the tags that get messed up.

As it's a multi-language site, I tried switching the language of the Tag to All, I tried switching the menu item language to All, but changing the language doesn't seem to change the behaviour.