That feature seems to be enabled in the Game's Settings itself. There are about 40+ games that you can enable that feature to use.
This AMD website for Fidelity FX shows what games uses Fidelity FX: https://www.amd.com/en/technologies/radeon-software-fidelityfx-supported-games
This is AMD Website for Fidelity FX: https://www.amd.com/en/technologies/radeon-software-fidelityfx
It mentions to disable in Radeon Settings Radeon Image Sharpening feature:
If you have one of the 40+ games go to the Game Settings to enable Fidelity FX with your RX6800 GPU Card.
Here is an example how to enable Fidelity FX in Capcom Monster Hunter World:
