SUMMARY
Ther are two packages: qt5-imageformats (ver. 5.15.2) and kimageformats (ver. 5.78.0) and they confuse DOlphin in terms of dislplaying thumbs some of the newer image formats. Additionaly there's another package: libavif (ver. 0.8.4) which further complcates thinsgs.

All of the are installed on my system, but regardless of the size (<3M) and the type of avif (8bpc, 8bpc yuv420, 10bpc-yuv444, 12bpc-pq, 12bpc-yuv444) Dolphin still can't display thumbs.

It used to be, that qt5-imageformats avif plugin did all the work. No furthers messing. This time kimageformats and qt5-imageformats have duplicate functionality, but Dolphin. doesn't use neither of them. 

Surprisingly gwenview can display thumbs and phtotos, but no thumbs in Dolphin. WHich one is Gwenview using? qt5-imageformats, libavif dircectly or kimageformats.

ONE SHARED LIBRARY.

{ps. there's another shared library with duplication: qt5-webkit and qt5-webrender}


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.20.5
KDE Frameworks Version: 5.78
Qt Version: 5.15.2

Dolphin: 20.12.1
libavif: 0.8.4
qt5-imageformats: 5.15.2