The media is 4K HDR, so when you start playback the TV is switched from SDR to HDR mode. This means everything is now in HDR mode so the OSD and (if you navigate out of playback) the Desktop will have saturated colours. Some platforms that Kodi supports (Android, Windows) can sometimes tonemap the OSD etc. to normalise the appearance, but on Linux and with low-power ARM SoC devices (as with Android) this is normally done using a dedicated hardware image processor function in the SoC (as doing it on the CPU is too intensive) and RPi boards do not have this capability (and even on devices that have it, software support in the kernel is rarely implemented in upstream Linux). There might be a possibility to improve things a little with shaders, but this is still something to be explored on the Kodi end.
In short, it's currently working as intended/capable/expected and this is not a bug.
Thank you for the answer, I have been reading in refernce to hdr and shading and the overlay as such.
This will explain why never noticed it on my FireCubes.
I was hoping the Pi5 hardware would be good enough to do this.
Looks like the Ugoos SK1 will be my set up in future when it comes.
I was nearly excited ot may have been fixed in a nightly.
But I know your expertise in this is amazing.
Thank you for your support.