Maybe there is an added class in the styling tab, or some styling where a text-transform: uppercase is applied to groupbox headers?
Hello Muhammed ;)
I've tried it myself in design mode
Are you sure you haven't added a building block with a group box containing a text element ? And that text element is by default in uppercase?
I could manage to add a groupbox but my groupbox is empty. I can therefore add a text element into it.
Let me know more details!