Hi Adams
Since no one have replied yet, here you go:
Your DS unit is HDR, while your Samsung Q70R is HDR10+, my guess is that the rocket fish is unable to isolate HDR10+ from edid signaling as it is not located where usual HDR10 signaling is located, so it fails to mix capabilities (assuming it mix anything in the first place)
Eventually using an EDID manager that copy your display EDID and insert it between Rocket fish output and your DS unit input would help. Not guaranteed since the splitter could still fail to retrieve HDR10+.
Doing the same using DS unit EDID and placing EDID manager between rocketfish output and Samsung QR70 could help too but you would then limit your Samsung to what DS is capable of... not optimal choice.
We do have solution that will work for you, Vertex2 can solve the issue or you can even drop DS if you go with Diva and our ambient light solution (which truly support HDR)