f9e39815 ("applets/batterymonitor: Improve for desktop systems") changed the plasmoid to use a plain icon instead of the CompactRepresentation if there are no batteries. While 471cc374 ("applets/batterymonitor: fix CompactRepresentation") brought the use of the CompactRepresentation back, the plain icon is still used.
For the case that there are no batteries and brightness controls available, it fell back to "battery", which has no symbolic variant. Change that to use the symbolic "video-display-brightness" as well.
Maybe it should just say "Power Management" in this case?
Plasma < 5.24.90: