This here is the solaar applet. If its loaded initially the icon is shown in the systemtray correctly. The icon changes once its loaded and detect a device. After the applet is fully loaded, the icon in the tray falls back to the default app.

Edit:
Found a workaround.
Starting solaar with -b solaar enforces the stock icon.
In niri its e.g.
spawn-at-startup "solaar" "-w" "hide" "-b" "solaar"

But when the icon is dynamic and it changes, the default icon pops up.