I am also a bit confused about what you describe your issue to be.
However, I often get the problem when parsing input that I don't really know when the
'transaction' (whatever that is) ends, only when a new one begins. In this case, when the new transaction starts, the first message print looks something like Serial.print("\nStart") so that the new line ('\n') for the last transaction is at the start of the new transaction, and everywhere else I just use .print(). The output will keep everything related on the same line and only start a new line when a new 'thing' is processed.
Hopefully this helps.