Honestly, I haven't gotten that far yet.
I am actually customizing the firmware that is written to the nRF52832 which I have, I have the Fanstel version which has a 20dBm amplifier on it.
But the UART example code they provide needs alot of tweaking to get it to work right for what I am doing. I had to play around with the defined MTU size, as well as the connection min//max interval time so I could get the throughput I needed. Plus the default firmware is crash-happy because of the NRF logging code which I commented out even though I am not in debug mode.
What I did notice which might also be your issue, is the Starter service gets destroyed, ALOT by Android 9. (which is what I am using) since the example targets SDK 26. So when that happens, bye bye connection. That may be whats occuring to you. But keep in mind this happens when my phone goes idle, and screen goes black from being idle.
But if I do a wakelock, and keep the app up and in the foreground, I dont have any problems. Yet.