Civicase add timeline but activities not shown
This looks like another casualty from the button UI changes. Have added https://lab.civicrm.org/dev/core/-/issues/2292. Can you try this small patch? https://github.com/civicrm/civicrm-core/pull/19356/files
The way that dropdown works is a bit weird - changing the value clicks a hidden button, and the way buttons submit their name to the form has changed, so it doesn't recognize it.