If you add a "Dark Box Text Lower Third" title, it works fine and renders fine.
Save the project, close resolve, reopen resolve and the project ... and the "dark box" (little animation on the left side) is gone ... doesn't show up while editing or render anymore.
(Resolve 17.1.1 Linux)
Resolve Studio - Ryzen 5800X3D - AMD RX6600 / NVidia RTX 4070 (switching between the 2) - Linux
If I edit the fusion composition for that title and go "fix" it, then it sticks and will work on save/load cycle. One of the Text element shader got switched from "Text" mode to "Char" mode on the first save.
Resolve Studio - Ryzen 5800X3D - AMD RX6600 / NVidia RTX 4070 (switching between the 2) - Linux