With Dolphin (KDE) 23.04.x and Trinity (TDE) R14.1.x running on my system
% trizen -Ss ffmpeg|grep -Eie installed
/usr/bin/doas: invalid option -- 'v'
usage: doas [-Lns] [-C config] [-u user] command [args]
/usr/bin/doas: invalid option -- 'v'
usage: doas [-Lns] [-C config] [-u user] command [args]
kde-unstable/ffmpegthumbs 23.04.2.r263.g8a8b7c2-1 [kde-applications kde-multimedia] [installed]
world/ffmpeg 2:6.0-8 [installed]
world/ffmpegthumbnailer 2.2.2-5 [installed]
world/ffmpegthumbs 23.04.2-1 [kde-applications kde-multimedia] [installed: 23.04.2.r263.g8a8b7c2-1]
world/qt6-multimedia-ffmpeg 6.5.1-1 [installed]
extra/ffmpeg 2:6.0-8 [installed]
extra/ffmpeg-audio-thumbnailer 1.1.1-3 [installed]
extra/ffmpegthumbnailer 2.2.2-5 [installed]
extra/ffmpegthumbs 23.04.2-1 [kde-applications kde-multimedia] [installed: 23.04.2.r263.g8a8b7c2-1]
extra/qt6-multimedia-ffmpeg 6.5.1-1 [installed]
aur/ffmpeg-audio-thumbnailer 1.1.1-2 [installed] [26 Feb 2023]
%
Strangely enough at this moment Dolphin media info+thumbnails actually works for me.
Try to double check in General-Previews, or as per previous advice reset Dolphin's settings.
Also, this might sound weird, but if Dolphin relies in some way on KDE frameworks it doesn't itself call, try running in the background another kde program, or use kwin_x11 as a window manager. nope, works fine without. You can also try pasting the output of env | grep -iE 'qt|kde' to see if it overruns anything.
Let me actually check on my thinkpad I have TDE installed on it, haven't ran it in a while :-)
LE: Yep I have thumbnails there too. Hmm.
That's been fixed by un-hiding previews, thanks! :)