I have the same problem with all 3 ceiling lights since the last firmware update (1.5.5_0181).
They all drop off WiFi many times per day (probably over 20). One dropped off permanently today, so I had to turn on/off wall switch in order to get it back online.
They worked flawlessly for the past 4-5 months and haven’t changed anything else on my side (same router, AP - Ubiquity pro).
I talked with few other users that have same issues since the last update so it must be software/firmware related.