XFCE 4.20 breaks comatibility with xrandr virtual screens

I am splitting my wide screen into two virtual screens which worked well with XFCE 4.18. After the upgrade to 4.20, several things broke.

Setup:

The screen is 3440x1440. I use the following commands to split the screen:

  xrandr --setmonitor LEFT 1520/350x1440/380+0+0 HDMI-1
  xrandr --setmonitor RIGHT 1920/450x1440/380+1520+0 HDMI-1
  xrandr --fb 3441x1440
  xrandr --fb 3440x1440

The first to actually splits the screen. The last two forces a refresh of the desktop by changing the resultion by 1 px (and back).

On XFCE 4.18 the following worked:

1. xfce4-panel recognizes both screens (LEFT and RIGHT). Binding a panel to a particular screen works
2. xfce4-panel "window list" with the option "show windows from all screens" unchecked shows the correct windows on each of the two virtual screens
3. It is possible to set a background image on both screens individually or to stretch one across both screens
4. Maximizing works independently on both virtual screens

On XFCE 4.20:

1. Still works, I can see both screen in the Options and bind the panels to each screen
2. Is not working anymore. With "show windows from all screens" unchecked no windows are shown.
3. Is not working anymore. The background set to the not-splitted screen is applied to both virtual screens. It's not possible to change the background as long as the screens are connected.
4. still works.

After downgrading xfce4-panel to 4.18, I got the functionality of the panel back. After downgrading xfdesktop to 4.18 I got the backgrounds back.