Problems like this can happen even to the largest manufacturers. There are reports of pixel phones losing WiFi connectivity after the recent Android 17 update if IPv6 is disabled. Good luck to those using phones of Google and Fairphone in the same network.
I would love to see a post-mortem after this is resolved to get an idea on what really went wrong, but don’t have high hopes on this.
Currently we do not know if Fairphone already identified the root cause or not. We do not even know how widespread the problem is. A Fritzbox seems reliably triggering the problem, but even this is guesswork.