If you are looking for a temporary workaround, at Elco’s suggestion I set my WiFi access point to use 20 MHz channels rather than 40 MHz. The brewpi spark has not disconnected from WiFi since.
If you are able to deal with the performance impact, at least until this issue is addressed in a software update for the spark, I would highly recommend adjusting the channel width on your WiFi router or access point.
Note: 40 MHz channels have higher data rates with 802.11n or 802.11ac, but are more susceptible to interference than 20 MHz channels.
@Elco, thanks for the tip!