I am running Manjaro-ARM-xfce-rpi4-20.12.1 on a Raspberry 3B+. The initial installation from a few days ago was working and the wallpaper was a basic ARM one which I never tried to change at all. At some point after that, the wallpaper changed to the default xfce, with only 4 variations of the same wallpaper available, no others. Not long after that, I realized that my taskbar had disappeared from the bottom of the screen. During that time I am not aware of having run any updates, I had only been installing a few other things to allow me to connect remotely with ssh and vnc.
could be that you changed for the taskbar to hide always, press the button to open whisker menu (typically on make pc’s keyboard the windows button) look up panel, go into it and change ‘automatically hide the taskbar’ to never press close and see if that works