I'm actually experiencing a similar, but also an exact opposite problem with the H115i Platinum I installed in my system a couple of weeks ago.
If I have my LED's set to full white, everything is fine. But if I turn OFF the lighting entirely/set it to black, the coolant temperature spikes by 2-3 degrees. Turn it back ON, drops back to normal.
Exact same thing happens if i try to set any other lighting option than a static white. Latest firmware + forced update, latest icue.
There's definitely something strange going on here.