Seems Windows 11 made updates on its Texteditor notepad.exe. The search functionality was changed (e.g. usability was improved by individual buttons for up and down search) as well as the undo functionality. But it seems that "undo" can not be reversed anymore. Up until the Windows 10 editor you could hit "undo" a second time to "redo" the undo. In Windows 11 this doesn't seem possible anymore due to the fact that it now supports multiple undos (in Win10 you could only undo your most recent change).
Am I missing something? If not, are there any other options (except for installing Notepad++ or other third-party notepad replacements - see my own answer)?
Side note: For issues regarding Notepad's search window usability (the problem that it covers the text content) please see this question. Thanks to Heinzi for the comment on this.
Side note: here's an interesting article about how the undo works in Notepad from 2013 - prior to Windows 11 which I found through this quesiton