On my FP3+ I also have the very similar problem that the phone disconnects from the wifi all the time (two or three times per hour). The difference: It always reconnects automatically after three seconds.
Which is a very bad thing for video calls, streaming, downloads, connections to Chromecast, wifi audio players, etc.
Finally I found a “solution” for this: If the 2.4-GHz access point is set to 802.11n-only, the wifi connection is stable.
But if the access point ist set to any mixed mode (for example: 802.11bgn or 802.11gn), then it continues to disconnect and reconnect all the time.
Of course, this is not a real solution, but a workaround. Maybe it helps you!
Let’s hope that Fairphone will fix this bug with an update very soon. Because I can not change the configuration of all access points I am connected to (public transport, at work, …).