Are you sure that you have the UL and not the UK? The UL is HDR400 so actually not really hdr. The only thing is that macOS doesn’t seem to realise this and with all the toggling there are four states that occur:

MacOS and monitor no HDR: “correct” colours and brightness MacOS HDR and monitor HDR: washed out colours / very low contrast macOS HDR and monitor No HDR: very strange picture macOS no hdr and monitor hdr: also washed out image