Are you using the GPULM that ships with the editor? Because Epic has abandoned that and it has not been updated in years, becoming increasingly more and more broken with every release.
If you are going to use GPULM you need to be using the fork here: Epic's GPUlightmass - #667 by yujiang.wang
The discoloration seems to be gone when you disable the denoiser which tells me that is most likely the cause, if that’s the case, using the version of GPULM in that link may help.
As for the loss of specular, I dont know. Like I mentioned earlier some loss of specular detail is inevitable because static lights don’t have any direct specular, without a static bake comparison between 5.4/5.6 it’s hard to know if anything is even wrong.