I have this problem since maybe April? 2015. I think this bug wasn't present in 4.2.2 but since 5.0 came along it's bad. 6.x made it worse - in 5.0 both signals were dropped occasionally when talking on the secondary SIM (the one without 4G data, regardless of which slot it's physically located in) but after about 30 seconds or so everything was back to normal, including mobile data. From version 6 mobile data is lost and you have to restart your phone to get back access to the internet (wifi is not affected)
I now reset the network settings but I don't think it would make any difference after going through 4-5 stock software upgrades and complete device resets every 1-2 months, restoring via official Sony software and also sending the device for repairs for other reasons where the service center restores factory software no matter what problems they are dealing with. They couldn't do anything about this particular issue, btw. I was hoping a major firmware upgrade (6.0) would fix the issue but it didn't so I'm very disappointed. This is a great phone with 1 SIM, but if I needed a one SIM phone I would have had tens of other options.
