Update:

These days I install Heroic Launcher with the rpm version from their site, yes, you have to manually download and install it, but it sees your standard mangohud configuration.

Original post:

When I play games through Steam I use MangoHud to display stats like FPS, CPU usage and so on. I use Goverlay to be able to configure MangoHud easily (it generates a config file).

I also use Heroic Launcher to play Epic Store games (the Flatpak version). Heroic can integrate MangoHud once you install the Flatpak version of it.

The Flatpak version don’t have the permissions to read your configuration file generated by Goverlay.

You just have to copy the config file in the right directory, in my case this command does the trick:

cp .config/MangoHud/MangoHud.conf .var/app/com.heroicgameslauncher.hgl/config/MangoHud/MangoHud.conf

Of course if you change your config you have to copy it again.

PS:
I have discovered the right folder looking at logs in Heroic (when you run a game it generates logs): it was even complaining because it wasn’t able to find the configuration files! LOL.