Do you have IGPU as primary display output in bios?
Ideal settings for AMD cards for fcpx are to set enable IGPU setting 64+ mb for DVMT allocated memory and then setting PEG or PCIE as primary display output. This allows the IGPU to help with decoding without robbing the performance of the AMD card. Video outputs should be connected to the AMD card.
If you are using IGPU for Heaven bench then the RX card is acting like a booster for the internal graphics, Geekbench scores use compute so it doesn't effect it like open gl apps.