... that area is transparent only when viewing a gallery in vertical mode, but not horizontal...
Looking at your screenshots, it looks like the caption area is visible in both horizontal and vertical modes. It's just the layout which (almost) hides the problem in vertical mode. You can see the top of the caption area overlapping the lower part of your image frame. The rest of the caption area just overlaps the gallery background so the problem cannot be seen in this area.
captionBackTopArea has a different default value for Small Screen Mode: rgba(0,0,0,0.3) instead of rgba(0,0,0,0).
However, explicitly setting it to rgba(0,0,0,0) does not seem to help.
This certainly looks like a bug. I'll notify the developers.
In the meantime, a suitable workaround should be to set captionBackTopColor to a fully transparent color other that black, as well as setting captionBackColor="rgba(0,0,0,0)". For example:
captionBackColor="rgba(0,0,0,0)"
captionBackTopColor="rgba(255,0,0,0)"
I hope this works OK.
Thank you for reporting this problem.