By the way, the Safe Frame, when enabled, appears before creating a rendering. While the black bars are visible, you can still change the camera perspective. This way you can define exactly what your rendering will turn out like, using the aspect ratio you've selected.
I hope this helps! Please let me know if there's anything left unclear.
Could the safe frame still show in the preview, please? It can be slightly grayed out just to indicate it's outside of the frame; however, it would still be easy enough to adjust views.
I tried the same frame again. It looks like when I click 'render' button once it shows the black bar. I can still adjust view. If I click 'render' bar one more time, then it actually exports a render image. Is that how it's supposed to work?