That is correct, as you are using captions (C608-1) instead of subtitles which has more options.
Recreate them in Transcript set to subitle default.
For 608/708, see the word "Background" toward the bottom of your screenshot? That dropdown selects the background color AND whether it is transparent (the default - does not show), Opaque, or Semi-transparent.
Ah missed that one.
All good Ann, the background colour dropdown Stan mentioned had no effect at all, no matter what colour I chose. I'd already explored that before this post.
Thanks for reporting back. Bottom line, you appear to have wanted regular subtitles with more styling options.
I did re-test, and background for 608 captions is working normally in PR 23.6. Select either Opaque or Semi-Transparent and then you will be the selected color.