Mangohud configuration with GOverlay not possible with flatpak Steam install

**Distribution name and version where applicable**
System: Kernel: 5.4.0-73-generic x86_64 bits: 64 compiler: gcc v: 9.3.0 Desktop: Cinnamon 4.8.6
wm: muffin dm: LightDM Distro: Linux Mint 20.1 Ulyssa base: Ubuntu 20.04 focal

**Flatpak info**
Flatpak 1.6.5

**Problem description**
I'm using linux mint as operating system. Steam was installed as flatpak. I have a problem when I tried to configure MangoHud.
Mangohud works in my games (I tested CSGO and Cities Skylines) but I am not able to modify/configure the MangoHud by GOverlay or even by manually editing MangoHud.conf.
When I checked sytem information in Steam I saw that in the graphics-details section the system reported "skipping config: /home/stefan/.config/MangoHud/MangoHud.conf [ not found ]" although this file exists! I have stored a modified configuration in this file and I am wondering about the error message that this file was not found.
I'm not sure but there are several links to "home/stefan/.local/share/Steam ...". This folder does not exist on my machine. I guess this is related to the flatpak install. All Steam files are located in "/home/stefan/.var/app/com.valvesoftware.Steam".

**Does this issue reproduce with native Steam**
No. I completely reinstalled Linux Mint on my machine and installed native Steam and MangoHud/GOverlay again. Now it works fine.