I've finally figured out why my BV9600 Pro keeps switching to the SSID on the 5GHz band even though the signal is almost out of range, rather than sticking to the same SSID on the 2.4Ghz band with a much stronger signal. It's because all the 2.4GHz SSIDs keep disappearing. I ran a WiFi analyzer app and the graph of the 5GHz SSIDs stays relatively flat across time. The 2.4GHz bands keep turning on and off every 10 or 20 seconds. The graph of 2.4GHz SSIDs stays flat for a while, dips to nothing, and then turns back on. No wonder the phone wants to stick with the 5GHz SSID, no matter how faint it is; at least it doesn't flip on and off.
Now, since no-one else has reported this, I can only assume that I have a phone with a faulty radio in it. How do I go about getting the phone replaced under warranty?