What version of xfce4-indicator-plugin and libindicator?
Seems like there is an underlying issue in Gtk:
https://bugzilla.gnome.org/show_bug.cgi?id=726030
Looks like atm there is no solution. The workaround is to move the panel/indicator away from the bottom of the screen.
*** Bug 12061 has been marked as a duplicate of this bug. ***
*** Bug 11090 has been marked as a duplicate of this bug. ***
I still have this issue in Xubuntu 22.04.