I've recently updated to version 20240329 of Tumbleweed which for me was the upgrade of KDE Plasma 5 to 6.

Since then, forgetting files via the start menu does not work as expected anymore.

Steps to reproduce
* open the "Application Menu" starter
* click "Recent Files" at the top to show the list of recent files
* right-click on one of the files and select "Forget File" from the context menu

Expected behavior
* the file gets removed from the list immediately (just like it did with KDE Plasma 5)

Actual behavior
* the file is still shown in the list

As it turns out the file does get forgotten because it is removed from the list in a fresh login session, so it seems to me it's just a UI refresh problem.
I still find it quite annoying because my use case often is to open files from the "Recent Files" list where I first have to remove several (to me) irrelevant entries before the document I'm looking for is shown.

I've also tested this with a fresh user account and the problem exists there as well.
Forgetting files does work as expected in "Application Starter" but not with "Application Menu".