Replying to provide additional info:
This is an upstream bug with KDE’s Oxygen theme, there is another recently closed thread reporting the same topic of thumbnails disappearing in Dolphin when the Oxygen theme was applied:
Related upstream bug reports:
504126 – kioworker thumbnail.so crashes with Oxygen on X11
504913 – kioworker (thumbnail plugin) crashes with SIGSEGV when using oxygen6 style on FreeBSD + Plasma 6
503799 – (X11-only) Thumbnails aren't working in general