Can't shutdown machine from Big Picture power menu

**Game information**
N/A

**Distribution name and version where applicable**

```sh
DISTRIB_ID="Arch"
DISTRIB_RELEASE="rolling"
DISTRIB_DESCRIPTION="Arch Linux"
```

**Flatpak info**
```
Flatpak 1.15.4
# flatpak --gl-drivers
default
host
```

**Problem description**

In Big Picture mode, going to the Power menu and selecting "System shutdown" does not work. Black screen, machine stays up.

```sh
# .xinitrc
#!/bin/sh

while ! [ "$(xrandr --listactivemonitors | wc -l)" -ge 2 ]; do
sleep 5
done

case "$(xrandr)" in
*'HDMI-1 connected'*) xrandr --output HDMI-1 --mode 1920x1080 ;;
*'HDMI-4 connected'*) xrandr --output HDMI-4 --mode 1920x1080 ;;
esac

xrandr > ~/xrandr.log 2>&1

export PATH="$PATH"

bspwm &
env > /home/moviuro/steam.log 2>&1
flatpak run -vvvvv --log-system-bus com.valvesoftware.Steam >> /home/moviuro/steam.log 2>&1
exec kodi
```

```
% tail ~/steam.log
System startup time: 13.77 seconds
BuildCompleteAppOverviewChange: 462 apps
RegisterForAppOverview 1: 3ms
RegisterForAppOverview 2: 4ms
Installing breakpad exception handler for appid(steam)/version(1679680416)/tid(728)
CAPIJobRequestUserStats - Server response failed 2
Installing breakpad exception handler for appid(steam)/version(1679680416)/tid(880)
Installing breakpad exception handler for appid(steam)/version(1679680416)/tid(893)
Installing breakpad exception handler for appid(steam)/version(1679680416)/tid(896)
Error org.freedesktop.DBus.Error.ServiceUnknown: org.freedesktop.DBus.Error.ServiceUnknown
Installing breakpad exception handler for appid(steam)/version(1679680416)/tid(901)
Installing breakpad exception handler for appid(steam)/version(1679680416)/tid(902)
```

NB: the `Error org.freedesktop.DBus.Error.ServiceUnknown: org.freedesktop.DBus.Error.ServiceUnknown` shows up when I'm trying to shutdown. Then log messages keep coming (`Installing breakpad...`)

```
% flatpak list
% flatpak list
Name Application ID Version Branch Installation
Steam com.valvesoftware.Steam 1.0.0.75 stable user
Proton (community build) com.valvesoftware.Steam.CompatibilityTool.Proton 7.0-5 stable user
Proton experimental (community build) com.valvesoftware.Steam.CompatibilityTool.Proton-Exp 7.0-20230208 stable user
Proton-GE (community build) com.valvesoftware.Steam.CompatibilityTool.Proton-GE 7.55-1 stable user
Freedesktop Platform org.freedesktop.Platform 22.08.10 22.08 user
i386 org.freedesktop.Platform.Compat.i386 22.08 user
Mesa org.freedesktop.Platform.GL.default 23.0.2 22.08 user
Mesa (Extra) org.freedesktop.Platform.GL.default 23.0.2 22.08-extra user
Mesa org.freedesktop.Platform.GL32.default 23.0.2 22.08 user
Mesa (Extra) org.freedesktop.Platform.GL32.default 23.0.2 22.08-extra user
Intel org.freedesktop.Platform.VAAPI.Intel 22.08 user
i386 org.freedesktop.Platform.VAAPI.Intel.i386 22.08 user
openh264 org.freedesktop.Platform.openh264 2.1.0 2.2.0 user
```

```
[...]
C362: -> org.freedesktop.UPower call org.freedesktop.UPower.EnumerateDevices at /org/freedesktop/UPower
B371: <- :1.13 return from C362
C363: -> org.freedesktop.UPower call org.freedesktop.UPower.EnumerateDevices at /org/freedesktop/UPower
B372: <- :1.13 return from C363
C364: -> org.freedesktop.UPower call org.freedesktop.UPower.EnumerateDevices at /org/freedesktop/UPower
B373: <- :1.13 return from C364
C1: -> org.freedesktop.DBus call org.freedesktop.DBus.Hello at /org/freedesktop/DBus
C2: -> org.freedesktop.DBus fake AddMatch for org.freedesktop.UPower
C3: -> org.freedesktop.DBus fake GetNameOwner for org.freedesktop.UPower
C4: -> org.freedesktop.DBus fake AddMatch for org.freedesktop.UDisks2
C5: -> org.freedesktop.DBus fake GetNameOwner for org.freedesktop.UDisks2
B1: <- org.freedesktop.DBus return from C1
B2: <- org.freedesktop.DBus signal org.freedesktop.DBus.NameAcquired at /org/freedesktop/DBus
B3: <- org.freedesktop.DBus return from C2
*SKIPPED*
B4: <- org.freedesktop.DBus return from C3
*SKIPPED*
B5: <- org.freedesktop.DBus return from C4
*SKIPPED*
B6: <- org.freedesktop.DBus return error org.freedesktop.DBus.Error.NameHasNoOwner from C5
*SKIPPED*
C6: -> org.freedesktop.ConsoleKit call org.freedesktop.ConsoleKit.Manager.Stop at /org/freedesktop/ConsoleKit/Manager
*HIDDEN* (ping)
B7: <- (no sender) return from C6
*REWRITTEN*
Error org.freedesktop.DBus.Error.ServiceUnknown: org.freedesktop.DBus.Error.ServiceUnknown
C365: -> org.freedesktop.UPower call org.freedesktop.UPower.EnumerateDevices at /org/freedesktop/UPower
B374: <- :1.13 return from C365
C366: -> org.freedesktop.UPower call org.freedesktop.UPower.EnumerateDevices at /org/freedesktop/UPower
B375: <- :1.13 return from C366
C367: -> org.freedesktop.UPower call org.freedesktop.UPower.EnumerateDevices at /org/freedesktop/UPower
B376: <- :1.13 return from C367
C368: -> org.freedesktop.UPower call org.freedesktop.UPower.EnumerateDevices at /org/freedesktop/UPower
B377: <- :1.13 return from C368
[...]
```

**Does this issue reproduce with native Steam**

Not tested