Panel on 2nd screen disappears when disconnecting/reconecting monitor

I have split my monitor into two using

  xrandr --setmonitor LEFT 1520/350x1440/380+0+0 HDMI-1
  xrandr --setmonitor RIGHT 1920/450x1440/380+1520+0 HDMI-1
  # force a refresh of wm
  xrandr --fb 3441x1440
  xrandr --fb 3440x1440

I have 2 panels, one with display set to "automatic", the other with display set to "LEFT".

The upgrade to xfce4-panel 4.20.4 introduced the following bug:

• when switching my monitor input to my other computer and back to mi primary, the 2nd panel disappears and must be restarted by xfce4-panel -r
• same when plugging out the hdmi plug an putting it in again

Downgrade to 4.20.3 and the bug is gone, so probably a regression between those.