Same WPA3 issue here with FP3 and Fritz!Box 7490 model (latest software build).
The WPA3-PSK security protocol has been working nominally on FP3 and router for more than a month now. Just stopped after today’s FP3 OS upgrade. After this upgrade, the Wi-Fi connection towards the router seems to be stuck using “WPA2-Personal” instead of “WPA3-Personal”. This is shown both on the router’s Wi-Fi page and on the Fairphone Wi-Fi network properties.