Install Qt6.5.1 gcc binaries on Ubuntu22.04 or RHEL9.0 including all Additional modules. Use example multimedia/screencapture, configure:

gcc_64/bin/qt-cmake -S /home/qt/RTA/Qt_6.5.1/Examples/Qt-6.5.1/multimedia/screencapture -B /home/qt/RTA/Qt_6.5.1/Examples/Qt-6.5.1/multimedia/screencapture/bld_gcc_64 -GNinja -DFFMPEG_DIR=/usr/local/FFmpeg-n6.0 

and build cmake --build . --parallel -j 8 in /home/qt/RTA/Qt_6.5.1/Examples/Qt-6.5.1/multimedia/screencapture/bld_gcc_64 both finish ok. Launch application, fails:

could not load multimedia backend "ffmpeg"
QtMultimedia is not currently supported on this platform or compiler. 

With Qt6.5.0 example was launched even there was no -DFFMPEG_DIR=/usr/local/FFmpeg-n6.0 in configure phase.

Works on SLES15_SP4 (no -DFFMPEG_DIR added to configure).

Fails on Ubuntu22.04 also if -DFFMPEG_DIR is not set in configure.