Well, its a little bit more obscure than what I thought.
After a fresh reboot, if I stop and restart the indi-weatherradio.service, /usr/share/weatherradio/bin/wr_rrd_update.py --verbose connects to the Weather Radio device and gives me only 0.000 weather values. If I start and then stop KStars/EKOS/indi with the Weather Radio driver I get expected weather values once I run /usr/share/weatherradio/bin/wr_rrd_update.py --verbose again. Makes me wonder if wr_rrd_update.py or what is in between wr_rd_update.py and the Weather Radio driver is missing some sensors or parameters configuration or initialisation, unless KStars and friends are run prior.
Currently running KStars 3.6.0 stable with indilib 1.9.7