Color space conversion in the UI composition is incorrect on DX12 HDR.
In the main() function In CompositeUIPixelShader.usf, you can see the following color space transform.
But it seems that the input of ColorLookupTable() must be linear color space. So it makes UI color incorrect on HDR.
Adding sRGB->Linear transform before the function fix this problem.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-117936 in the post.