Log in / Log out from standalone UI5 application

I have build an UI5 application that is deployed to an onpremise SAP System. When I start the application the SAP System automatically asks for a user log on. For the log off from the same application using the SICF knot /sap/public/bc/icf/logoff the system responds with a successful log off page. But when I reload the page the user is still logged on.

How can I provide a "real" log off from the system? It seems that there is somehow a session saved on the server side.

As a side note I cannot see any session in the SM04 after the log in.

Thanks for your help!