I´m afraid that I have found a bug…

Yes and no.

• I go to http://forum.derletztekick.com as a guest-user
  
• I click "gesamten Thread öffnen" on a thread that I haven´t visited yet
  
• I go back to the frontpage
  
• The thread is still not marked (colored) as visited

Same problem in "letzte Einträge".

This is, as far as I know, an always existing (mis)-behaviour.

No, I have a version 2.3 live here: http://www.jernbanen.dk/forum/
where it works as expected. Tested in Chrome and Edge.