RS485 Multidrop Issue
You should be able to prove that the unwanted node is interrupting things by using your scope - check the data out line from its MCU isn't bursting into life when the wanted node is replying. I'm beginning to suspect that the reply from the 1st node contains a byte that looks like an address for the 2nd node.


  Yes, the unwanted node seems to be transmitting when the other node
  replies to the master.


This is a common problem when designing data links. Usually what can be done is to send a first byte header that is unique and never contained in any payload data. Following that header you put the target address of the slave thus any slave can only consider replying when the unique header plus address are received. Following those bytes will be the payload data and finally an error check byte (or two).

To avoid payload data looking like the unique header use a technique known as byte stuffing OR, wait until the checksum is received in order to verify that a correct message has been received and not some snippet of data from another slave.

You can also look for breaks between transmissions in order to re-sync when looking for an address. You should also consider not transmitting blindly when another byte is being sent - a break of at least 10 bits is usually sufficient to get a more robust system.