It looks like the soundbar capabilities aren't being exposed to the RPi5 through EDID data on the HDMI connection. The debug log clearly shows only FL/FR speakers are available; hence Kodi is downmixing 6.0 media to 2.0 to match the advertised layout. If pass-through is used, the stream is not decoded (for downmix) and magic on the TV side results in the soundbar working.
I'd be looking into TV settings for eARC? options to ensure the Soundbar EDID is presented to the RPi5, then things should work. If nothing exists the workaround might be to temporarily connect the RPi5 directly to the soundbar, run "edid create" to capture the EDID data, then reconnect the RPi5 to the TV again. On reboot the RPi5 will see the cached Soundbar data not the live TV data.