A popular use-case is continuous data streaming, where UART writes need to be called consecutively.

When implementing an application where uart writes are called inside the callback, it is seen that the driver returns UART_TRANSFER_STATUS_ERROR_INUSE for the second transaction. 

Root Cause:

The UART driver clears writeTrans AFTER the callback returns (not before), so calling UART_write() from within the callback fails with ERROR_INUSE because the driver still thinks a transfer is in progress.