SUMMARY
The cb7 thumbnail generation support that landed in 19.12 (to address https://bugs.kde.org/show_bug.cgi?id=410696) works, but only if LZMA, LZMA2 or BZip2 compression is used. No thumbnails are generated if the cb7 archive is compressed with Deflate, Deflate64, Copy or PPMd.

STEPS TO REPRODUCE
1. Compress an image (of a supported format such as JPG or PNG) with `7z` into a cb7 archive:
 for m in {lzma,lzma2,ppmd,deflate,deflate64,copy,bzip2}; do 7z a -m0=$m test-$m.cb7 filename.png; done
2. Enable previews in Dolphin
2. Navigate to the directory containing the new files in Dolphin

OBSERVED RESULT
Thumbnails are shown for the cb7 files compressed with LZMA, LZMA2 and BZip2, but a generic icon for those compressed via Deflate, Deflate64, Copy, and PPMd.

EXPECTED RESULT
Thumbnails are shown for all cb7 files, regardless of compression used.

SOFTWARE/OS VERSIONS
Windows               : n/a
macOS                 : n/a
Linux/KDE Plasma      : 64-bit, 4.19.86-gentoo
KDE Plasma Version    : 5.17.5
KDE Frameworks Version: 5.67.0
Qt Version            : 5.14.1

ADDITIONAL INFORMATION

The same issue arises when opening the created cb7 files with Okular; it will not open cb7 files unless they are compressed with LZMA, LZMA2 or BZip2. Presumably a shared component (K7Zip?) is causing this. Ark and MComix read cb7 files using any compression methods, as expected.

7-Zip 16.02 (64-bit)
Dolphin 19.12.3
kio-extras 19.12.3