Upon upgrading to Plasma 6.7.0, Dolphin’s autocomplete functionality in the address bar no longer seems to exist. I was about to file a bug report, but I’m not sure if this is intentional.
It is a bug. You may have enabled the Plasma virtual keyboard with the option ‘Show a pop-up window when a key is held down’.
See https://bugs.kde.org/show_bug.cgi?id=521789
You can disable the Plasma on-screen keyboard (Control Centre → Keyboard → On-screen keyboard) or untick ‘Alternative characters’ (Control Centre → Keyboard → On-screen keyboard (OSD)).