Regression: panel is not shown on external display after reconnecting

This is a regression observed in xfce4-panel-4.20.4. I have two panels configured for my laptop, one for the primary display, and one for the external (HDMI) display. If I start xfce4-panel while the external display is connected, both panels display as expected. However, if I then disconnect the external display and reconnect it, the second panel for the external display is no longer shown and I have to restart xfce4-panel to get the second panel back. Running with PANEL_DEBUG=1 showed now differences in the log output between the two versions after the monitor HDMI-A-0 not found, hiding window line after reconnecting the external display, aside from the hex values.