I can confirm that behavior.
When I edit a logfile with Notepad++, it adds an CR at the end and the log entry appears in checkmk GUI.
Somehow if I use the Windows-Builtin Notepad it doesn’t add the CR at the end and the log-entry doesn’t appear in checkmk-GUI.
Could be an explaination for the different outcome of my first two tries mentioned 13 days ago.