Hello Stephen,
If you're not handling the incoming data with g_uart1.p_api->read, then eventually you'll run out of the software buffer. By default, incoming data is transferred into driver-level buffer using DTC on RXI trigger and then generate UART_EVENT_RX_CHAR event in the UART callback. Using read function will pop these bytes to the application-defined buffer you provide a pointer to when calling read. Failure to do so will result in the bytes being overwritten before you can retrieve them.
Regards