So What's the problem ?
You don't have links to the hardware you're using, so I have to guess about the problem. It might be that the bus doesn't have a defined state if none of the devices is sending (no bus driver is activated). I have pull up/down resistors (only once in the whole bus) to define a state in this case. It might be that your RS-485 module includes this but I simply don't know.
It would explain why the transfer of the acutal message is correct but around the message you receive random data.
Do you have a link to the module you're using?