background turns solid black on new workspace

Description:
On gnome 3.34.1, when getting to a new workspace, guake suddenly has a solid (non-transparent) black background instead of the expected setup.

It is however as expected on startup.

Then, after just opening then closing the preferences window (even without touching anything), the background turns back as expected, but not for further opened workspaces.

guake --support:
```
$ guake --support
Guake Version: 3.6.3
Vte Version: 0.58.2
Vte Runtime Version: 0.58.2
--------------------------------------------------
GTK+ Version: 3.24.12
GDK Backend: GdkX11.X11Display
--------------------------------------------------
Desktop Session: ubuntu
--------------------------------------------------
Display: :0
RGBA visual: True
Composited: True
* Monitor: 0 - CMN eDP-1
* Geometry: 1366 x 768 at 0, 0
* Size: 344 x 193 mm²
* Primary: True
* Refresh rate: 60.00 Hz
* Subpixel layout: unknown

```