Certainly an interesting vulnerability, but one you shouldn't worry about.

If you do really care about sandbox security, the first thing I would recommend doing is globally blocking filesystem access to anywhere in your $HOME that runs script code, such as:

• bash files like ~/.bashrc and ~/.bash_profile
• ~/.local/bin and ~/bin
• ~/.ssh

I have a script that I use to control flatpak overrides and I do something like this:

# paths to block
GLOBAL_RESTRICTION_PATHS=(
    "~/.bash_logout"
    "~/.bash_profile"
    "~/.bashrc"
    "~/.profile"
    "~/.ssh"
    "~/.zshenv"
        "xdg-config/zsh"
    "~/.local/bin"
    "xdg-config/systemd"
)

# globally block these paths
for path in "${GLOBAL_RESTRICTION_PATHS[@]}"; do
    flatpak --user override --nofilesystem="$path"
done

# but allow some apps like text editors to access them
for path in "${GLOBAL_RESTRICTION_PATHS[@]}"; do
    flatpak --user override --filesystem="$path" org.gnome.TextEditor
done