SUMMARY
In an X11 session, the standard Network speed widget on my system does not display anything. The line chart is not drawn, and the up/down speeds are not reported (there are just  two small cyan and orange rectangles on the left-bottom part). It feels like no network interface is discovered: indeed, editing the set of displayed sensors, in the "Network device" set there is only the "All network devices" subset. I'm pretty sure it worked just after I updated from 5.19.5 to 5.20.0, with the sensors for the individual network interfaces being available, but it stopped working shortly after, even before upgrading to 5.20.1. I can't say if this is due to updating another system package in the middle, but anyway now it's broken.


STEPS TO REPRODUCE
1. add a Network speed widget to the desktop
2. verify that the line chart does not display any line, just the level lines having all a "0 B/s" label


SOFTWARE/OS VERSIONS
Linux: Archlinux
KDE Plasma Version: 5.20.1
KDE Frameworks Version: 5.75.0
Qt Version: 5.15.1

ADDITIONAL INFORMATION
Bug observed in an X11 session, didn't try with Wayland.