"Then I tried the same device, this time configured as a master, and it works with the unit."
To clarify: that description is a USB stick with PC software as master receiving reverse direction messages from the nRF52832 slave?
If that is so, then your phone app should be working with the nRF52832 slave.
I recommend sniffing the packets going between the phone and nRF52832 using ANTwareII configured in continuous scanning mode. You will see the messages going in both directions.
Test with the working case of PC master and nRF52832 slave first and observe the forward and reverse direction messages. Then test the non-functional case of phone and nRF52832. Check that you still see both forward and reverse direction messages. The forward direction messages will occur slightly before the reverse direction messages, followed by a gap of ~1 channel period to the next forward direction message.