I understand that that's what you believe. It is, however, not what is happening here.
I am running Subsurface here on Ubuntu 20.04 just fine and it happily accesses the ~/.subsurface directory
Are you running a snap or AppImage or something? The description in your original email is nonsensical: "Subsurface 4.9.4 latest/candidate (latest/stable doesn't work, (qt5 error))"
The recommended way to install Subsurface for Ubuntu would be from our PPA
sudo apt-add-repository ppa:subsurface/subsurface
sudo apt-get update
sudo apt-get install subsurface
/D