question

Ok, it was my own fault. I implemented the Piko 8.3 inverter script inspired by Fabian Lauer's Shelly3EM implementation here:
https://github.com/fabian-lauer/dbus-shelly-3em-smartmeter

But I was writing the Piko energy data (like in original implementation) to
/Ac/Energy/Reverse registers. Obviously, following the Victron terminology, inverters need to send Energy values to /Ac/Energy/(Lx)/Forward registers.
Now, the picture looks like:

BTW, I decoupled the script running on Cerbo and writing data to dbus from that one retrieving the Piko data as my Piko doesn't respond to http requests 1-2 times a day.
The latter runs on a Raspi, queries the Piko html page and transforms the data to json, finally writes it to a file (Shelly EM3 style):

{"wifi_sta":{"connected":true,"ssid":"ede2wlan","ip":"192.168.1.4","rssi":-38},"cloud":{"enabled":true,"connected":true},"mqtt":{"connected":false},"time":"12:43","unixtime":1692355381,"serial":7416,"has_update":false,"mac":"C45BBE785136","cfg_changed_cnt":0,"actions_stats":{"skipped":0},"relays":[{"ison":false,"has_timer":false,"timer_started":0,"timer_duration":0,"timer_remaining":0,"overpower":false,"is_valid":true,"source":"input"}],"emeters":[{"power":1279 ,"pf":-1.00,"current":5.00,"voltage":235,"is_valid":true,"total":2623.33,"total_returned":0},{"power":1263,"pf":-1.00,"current":5.00,"voltage":232,"is_valid":true,"total":2623.33,"total_returned":0},{"power":1271,"pf":-1.00,"current":5.00,"voltage":235,"is_valid":true,"total":2623.33,"total_returned":0}],"total_power":3813,"fs_mounted":true,"update":{"status":"idle","has_update":false,"new_version":"20220209-094824/v1.11.8-g8c7bb8d","old_version":"20220209-094824/v1.11.8-g8c7bb8d"},"ram_total":49440,"ram_free":27940,"fs_size":233681,"fs_free":154867,"uptime":1000}

The former contacts the webserver running on Raspi and gets the json file. In case the Piko does not respond to the http request (Raspi always does), there is at least the old version of json file served to Gerbo request.

Question closed ;-)